BLKMBNTS 


OF 


STATIC  ELECTRICITY 


WITH 


FULL   DESCRIPTION    OF  THE   HOLTZ  AND  TOPLER   MACHINES 
AND  THEIR    MODE  OF  OPERATING. 


By   PHILIP  ATKINSON,   A.M.,  Ph.D. 


NEW  YORK: 
W.   J.   JOHNSTON,     PUBLISHER, 

168-177    POTTER    BUILDING. 
1887. 


Copyright,  1886, 
BY  W.  J.  JOHNSTON. 


INTRODUCTION. 


IN  this  treatise  the  principles  of  electricity  are 
presented  untrammeled,  as  far  as  possible,  by  mathe- 
matical formulae,  so  as  to  meet  the  requirements  of  a 
large  class  who  have  not  the  time  or  opportunity  to 
master  the  intricacies  of  formulae,  which  are  usually  so 
perplexing  to  all  but  expert  mathematicians. 

This  class  includes  those  whose  knowledge  of 
electricity  is  limited  to  .the  practical  details  of  teleg- 
raphy, telephony,  and  electric  lighting;  also  those 
among  the  liberally  educated,  who  desire  to  review 
electric  science  in  the  light  of  recent  investigation; 
and  those  who  wish  to  study  its  elementary  principles, 
preparatory  to  a  more  extended  course,  which  shall 
embrace  all  the  details  of  electric  measurement  arid 
electric  engineering. 

The  original  plan  included  dynamic  as  well  as  static 
electricity,  embracing  its  practical  application  to  the 
arts ;  but  it  was  subsequently  thought  best  to  confine 
the  present  work  to  static  electricity  alone,  to  meet 
the  wants  of  those  who  are  less  familiar  with  its  prin- 
ciples than  with  those  of  dynamic  electricity,  and  to 


1 V  IX  TROD  UC  TWN. 

reserve  the  consideration  of  the  latter  for  a  separate 
volume. 

Care  has  been  taken  to  avoid  the  introduction  of 
new  matter  before  the  student  was  prepared  for  it ; 
hence  it  was  thought  best  that  there  should  be  a 
thorough  examination  of  elementary  principles  before 
introducing  complicated  apparatus,  the  construction 
and  operation  of  which  depends  on  those  principles. 

The  theory  assumed  is,  that  electricity  is  one  of 
the  forms  in  which  energy  manifests  itself;  that  its 
nature  is  not  changed  by  the  means  emplo}Ted  to 
generate  it,  and  that  the  various  terms,  positive, 
negative,  static,  dynamic,  express  certain  conditions  and 
relations  in  which  this  manifestation  occurs,  and 
not  different  kinds  of  electricity. 

The  author  takes  pleasure  in  acknowledging  his 
obligations  to  Elisha  Gray  for  the  use  of  tables,  giving 
the  results  of  observations  on  earth  currents,  made 
under  his  direction  on  the  Postal  Telegraph  line;  also 
to  Ferguson,  Gordon,  Silvanus  P.  Thompson,  Noad 
uiicl  Deschanel,  from  whose  excellent  works  valuable 
assistance  has  been  obtained,  though  he  has  felt 
compelled  to  dissent  from  some  of  their  views. 

The  views  here  expressed  are  the  result  of  many 
years'  experience  in  the  class  room,  the  lecture  room, 
and  the  laboratory,  and  were  adopted  only  after  the 
most  rigid  test  of  actual  and  oft  repeated  experiment. 
And  some  of  the  more  important  apparatus  described 


7,V  TROD  U(.  'TION.  V 

is  of  the  author's  own  manufacture,  constructed  in 
strict  accordance  with  electric  principles,  verified  by 
his  own  experiments. 

While  humbly  following  the  great  pioneers  in 
electric  science,  who  have  hewed  waymarks  on  the 
rocks,  the  author  will  rest  content  if  he  has  left  some 
foot-prints  on  the  sands,  which  may  serve  to  guide  the 
wayfarer  till  obliterated  by  the  coming  waves  of 
progress. 

The  impartial  criticism  of  teachers  and  electricians 
is  especially  requested,  that  faults  and  errors  may  be 
corrected  in  future  editions. 

PHILIP  ATKINSON. 

CHICAGO,  June,  1886. 


CONTENTS. 


CHAPTER    L 

PAGE 

ATTRACTION  AND  REPULSION, 1 

CONDUCTORS  AND  NON-CONDUCTORS,      ...  4 

QUANTITY  AND  INTENSITY, 6 

STATIC  ELECTRICITY  DEFINED, 8 

CHAPTER  IT. 
ELECTRIC  POTENTIAL 10 

CHAPTER   III. 
THE  NATURE  OF  ELECTRICITY, 23 

CHAPTER   IV. 
INDUCTION, 43 

CHAPTER  V. 
ELECTRIC  DISTRIBUTION  AND  CONDENSATION,       .        .  55 

CHAPTER  VI. 
ACCUMULATORS, 72 


Vlll  CONTENTS. 

CHAPTER  VII. 
ELECTRIC  GENERATORS. — 

THE  ELECTROPHORUS  AND  FRICTIONAL  MACHINE,          92 


CHAPTER   VIII. 
ELECTRIC  GENERATORS. — 

THE  HOLTZ  AND  TOPLER  MACHINES,  .        .        .          108 

CHAPTER   IX. 
EXPERIMENTS  WITH  THE  TOPLER  MACHINE,  .         .         .  125 

CHAPTER  X. 
ELECTRIC  TRANSMISSION  IN  VACUA,       ....          146 

CHAPTER  XI. 
ELECTROMETERS, 155 

CHAPTER  XII. 
THE  ELECTRICITY  OF  THE  EARTH  AND  ATMOSPHERE. — 

POTENTIAL  AND  EARTH  CURRENTS,      .         .         .  175 

CHAPTER   XIII. 
THE  ELECTRICITY  OF  THE  EARTH  AND  ATMOSPHERE. — 

THE  AURORA, 190 

CHAPTER   XIV. 
THE  ELECTRICITY  OF  THE  EARTH  AND  ATMOSPHERE. — 

LIGHTNING  AND  THUNDER, 207 


ELEMENTS 

OF 

STATIC   ELECTRICITY, 


CHAPTER  I. 

ATTRACTION  AND  REPULSION  —  CONDUCTORS  AND 
NON-CONDUCTORS — QUANTITY  AND  INTENSITY- 
STATIC  ELECTRICITY  DEFINED. 

ATTRACTION  AND  REPULSION. — Amber,  called  in 
Greek  rfaxroov,  was  known  to  the  ancients  to  acquire, 
when  rubbed,  the  power  of  attracting  light  bodies; 
hence  this  property,  now  known  to  belong  to  all  sub- 
stances, has  received  the  name  of  electricity.  The 
earliest  conception  of  electricity,  then,  was  that  of 
force,  and  the  latest  discoveries  sustain  this  view. 

Electricity  may  be  generated  by  various  simple 
methods,  as  follows: — Let  a  spoon  be  balanced  on  the 
edge  of  a  cup,  and  an  ebonite  (hard  rubber)  knife- 
handle,  rubbed  on  a  woolen  or  silk  fabric,  be  held 
near  it,  and  the  spoon  will  be  attracted.  Substitute  for 
the  knife-handle  a  stick  of  sealing-wax,  a  lamp-chim- 
ney, or  a  paraffin  wax-candle,  rubbed  in  the  same  way, 
and  the  spoon  will  be  attracted  by  each  of  them. 

These  different  substances  may  be  multiplied,  and 
different  rubbers  used,  but  it  will  be  found  that  the 


Z  ELEMENTS  OF  STATIC  ELECTRICITY. 

attractive  force,  though  variable  in  intensity,   is  com- 
mon to  all. 

The  balanced  rod,  represented  in  Fig.  1,  will  be 
found  more  convenient  for  these  experiments  than  the 
balanced  spoon.  It  consists  of  a  round  wooden  rod, 
about  twenty  inches  long,  and  half  an  inch  in  diameter, 
with  the  ends  rounded  or  terminating  in  balls.  It  is 
pivoted  at  the  center  on  a  point,  and  may  be  mounted 
on  a  stand,  oryon  a  bottle  with  a  pin  through  the  cork, 
and  made  to  revolve  rapidly  by  the  force  of  attrac- 
tion, following  any  of  the  electrified  bodies  already 
mentioned  wrhen  held  near  it,  as  represented. 


Fig.  1— The  Balanced  Rod. 

A  more  sensitiveinstrument  for  investigations  of  this 
class  is  represented  in  Fig.  2,  and  known  as  the  pith- 
ball  electroscope;  the  name  electroscope  being  derived 
from  the  Greek  cxoTteco,  to  see,  ifaxrQov,.  electricity.  It 
is  constructed  as  follows  :  A  small  brass  rod,  bent  at 
right  angles,  has  its  short  arm  inserted  into  an  ebonite 
stem  attached  to  a  wooden  base,  giving  the  instrument 
a  vertical  height  of  about  Hi  inches.  The  horizontal 
arm  is  about  8  inches  long,  and  terminates  'in  a  small 
brass  ball.  From  this  arm  two  pith  balls,  each  about 
half  an  inch  in  diameter,  are  suspended  by  silk  threads. 


ATTRACTION  AND  REPULSION.  3 

Let  the  pith  balls  be  separated  at  the  points  of  sus- 
pension, so  that  when  they  hang  vertically  a  consider- 
able space  shall  intervene  between  them,  and  let  a 
stick  of  sealing-wax,  previously  electrified  by  friction, 
be  brought  near  one  of  them  ;  the  ball  will.be  attracted 
to  the  wax,  and,  after  a  momentary  contact,  repelled. 
Follow  it  with  the  wax,  and  it  continues  to  recede  as  if 
pushed  back  by  some  invisible  barrier. 

Now  let  the  other  pith 

ball  be  moved  near  this  /  »v 

one,  and    they  will   be  '    '• 

attracted  to  each  other, 
and,  after  contact,  re- 
pelled ;  the  lines  of  sus- 
pension showing  diver- 
gence in  each  direction 
as  represented. 

Let  the  electrified  wax 
be  again  brought  near, 
and  each  ball  is  repelled 
by  it,  so  that  when  it  is      Fi«'  2~T"e  Pk"-Ba!1  Electroscope, 
placed  between   them,  they  are  driven  further  apart ; 
but  let  any  non-electrified  body  be  brought  near  and 
they  are  attracted  to  it. 

If  each  of  the  balls  be  separately  electrified  by  the 
wax,  and  they  are  then  brought  near  each  other,  they 
will  show  mutual  repulsion  without  previous  attraction. 
"From  this  series  of  phenomena  we  learn,  first,  that 
electrified  bodies  not  only  attract  non-electrified  bodies, 
as  already  shown,  but  communicate  electricity  to  them 
by  contact ;  and,  secondly,  that  bodies  electrified,  either 
by  each  other  or  from  the  same  source,  show  mutual 
repulsion. 


4  ELEMENTS  OF  STATIC  ELECTRICITY. 

The  first  fact  was  shown  when  the  pith  ball,  after 
contact  with  the  wax,  attracted  and  electrified  the 
other  pith  ball ;  and  the  second  fact  by  the  repulsion 
of  the  pith  ball  from  the  wax  after  contact ;  then  of 
ther  two  pith  balls  from  each  other  and  from  the  wax, 
after  contact ;  and  finally  by  the  mutual  repulsion  of  the 
balls,  without  previous  attraction,  after  being  separately 
electrified  by  the  wax. 

This  series  of  phenomena  may  be  produced  by  using 
a  glass  or  ebonite  rod,  or  any  of  the  substances  already 
mentioned,  as  well  as  by  the  sealing-wax  ;  showing  that 
repulsion  as  well  as  attraction  is  a  property  common  to 
all  electrified  bodies. 

CONDUCTORS  AND  NON-CONDUCTORS. — Pursuing  our 
investigation,  new  properties  are  developed.  It  is 
found  that  while  certain  substances,  as  glass,  ebonite, 
and  sealing-wax,  show  electric  qualities,  others,  as  brass, 
iron,  and  copper,  apparently  do  not  show  such  qualities. 
This  led  to  the  old  division  of  all  substances  into 
electrics^  a  term  applied  to  the  former,  and  non-electrics, 
applied  to  the  latter. 

But  more  thorough  investigation  has  proved  that 
electricity  may  be  generated  by  friction  on  the  brass, 
iron,  and  copper,  as  well  as  on  the  glass,  ebonite,  and 
sealing-wax  ;  but  that,  when  generated  on  bodies  of  the 
former  class,  it  is  instantly  distributed  over  the  entire 
body,  and  escapes  to  the  earth  unnoticed,  if  the  body 
be  held  in  the  hand,  while,  when  generated  on  bodies 
of  the  latter  class,  it  is  not  so  distributed,  and  does  not 
pass  off  in  this  way;  bodies  of  the  former  class  allowing 
free  electric  movement,  over  the  surface  or  through  the 
mass,  while  those  of  the  latter  class  resist  such  move- 
ment. 


CONDUCTORS  AND  NON-CONDUCTORS.  5 

To  make  this  evident,  let  a  short  brass  rod,  of  about 
quarter  inch  diameter,  terminating  in  a  ball,  be  fitted  to 
an  ebonite  handle,  as 
represented  by  Fig.  3. 
Let  the  brass  be  rubbed 

briskly  on  woolen,  silk,          FiS-  3-The  Insulated  Metal  Rod. 

or  india  rubber,  holding  the  instrument  by  the  handle, 
and  it  will  attract  and  repel  the  pith  balls  in  the  same 
way  as  the  other  electrified  substances  already  used. 
Copper,  iron,  or  any  other  metal  may  be  substituted 
for  the  brass  with  the  same  result. 

Kepeat  the  experiment,  allowing  the  metal  to  touch 
the  hand,  and  the  electric  qualities  disappear.  This 
shows  that  in  the  first  experiment  the  electricity  was 
retained,  because  it  could  not  pass  through  the  ebonite 
handle ;  while,  in  the  second,  it  passed  off  through  the 
hand. 

The  results  obtained  by  experiments  of  this  kind  led 
to  the  abandonment  of  the  doctrine  of  electrics  and 
non-electrics,  and  the  classification  of  all  bodies  as  con- 
ductors or  non-conductors. 

Experiment  proves  that  all  substances  conduct  elec- 
tricity, and  that  they  all  offer  a  certain  amount  of 
resistance  to  its  passage.  But  it  is  found  that  the 
relative  proportions  of  conductivity  and  resistance  vary 
greatly  in  different  substances.  In  some  the  conduct- 
ivity is  largely  in  excess,  and  they  are  called  conductors ; 
in  others  the  resistance  is-  largely  in  excess,  and  they 
are  called  non-conductors.  Between  these  extremes 
there  are  all  degrees  of  variation ;  so  that  in  some  sub- 
stances the  two  properties  are  almost  equally  balanced. 
Hence,  since  no  exact  rules  can  be  given,  we  distinguish 
the  two  classes  by  saying  that  a  CONDUCTOR  is  any  sub- 


6  ELEMENTS  OF  STATIC  ELECTRICITY. 

stance  of  such  low  resistance  that  it  can  be  used  practi- 
cally for  the  transfer  of  electricity;  and  a  N OX-CON- 
DUCTOR is  any  substance  of  such  high  resistance  that  it 
can  be  used  practically  to  prevent  such  transfer. 

LIST  OF  CONDUCTORS  AND  NON-CONDUCTORS. — The 
principal  conductors  are  the  metals,  silver  and  copper 
being  the  best.  Among  the  partial  conductors  are  the 
different  varieties  of  carbon,  including  coal,  charcoal, 
and  graphite ;  the  acids,  saline  solutions,  water,  vegeta- 
bles, and  animals. 

The  principal  non-conductors  are  caoutchouc,  gutta- 
percha,  sulphur,  and  their  compound,  known  as  hard 
rubber,  vulcanite,  or  ebonite ;  dry  air,  paraffin,  shellac, 
amber,  resin,  glass  when  free  from  metallic  substances, 
mica,  silk,  fur,  wool,  hair,  feathers,  bisulphide  of  carbon, 
petroleum,  and  oil  of  turpentine. 

Among  the  partial  non-conductors  are  porcelain, 
baked  wood,  paper,  and  leather. 

INSULATOR  DEFINED.  —  When  a  non-conductor  is 
used  in  connection  with  a  conductor  to  confine  elec- 
tricity within  certain  limits,  it  is  called  an  insulator; 
and  the  conductor  on  which  the  electricity  is  confined, 
or  to  be  confined,  is  said  to  be  insulated ;  as  a  metal 
placed  on  a  glass  or  ebonite  support,  a  copper  wire 
wrapped  with  silk  or  wool. 

QUANTITY  AND  INTENSITY. — Electric  quantity  and 
intensity  are  similar  to  the  quantity  and  intensity  found 
in  other  more  familiar  forms  of  energy.  The  intensity 
of  any  form  of  energy,  other  things  being  equal,  is  in- 
versely proportional  to  the  mass  of  the  body  in  which  it 
is  developed.  A  few  strokes  of  a  hammer  on  a  small 
piece  of  iron  placed  on  an  anvil  will  raise  its  temper- 
ature to  a  burning  heat;  while  the  same  number  of 


QUANTITY  AND  INTENSITY.  7 

strokes  on  a  large  mass  of  iron  will  produce  but  very 
slight  change  of  temperature.  The  quantity  of  muscular 
energy  expended  is  the  same  in  each  case,  but  the  inten- 
sity of  heat  energy  produced  varies  inversely  as  the 
mass. 

The  intensity  varies  also  as  the  resistance.  A  small 
piece  of  wood  held  in  the  flame  of  a  lamp  is  quickly 
ignited  at  the  end  in  the  flame ;  while  the  end  held  in 
the  hand  shows  no  perceptible  change  of  temperature. 
But  a  brass  rod  of  the  same  size,  similarly  held  for  the 
same  length  of  time,  becomes  too  hot  for  the  hand  long 
before  the  end  in  the  flame  is  hot  enough  to  ignite 
wood. 

In  the  wood,  the  intensity  rises  rapidly  at  the  end 
held  in  the  flame,  because  the  resistance  prevents  dis- 
tribution of  heat  through  the  mass.  But  the  low 
resistance  of  the  brass  permits  the  rapid  distribution 
of  the  same  quantity  of  heat  through  its  mass  ;  so  that  the 
intensity  at  the  end  in  the  flame  is  much  less  than  that 
of  the  wood. 

In  kindling  a  fire  of  anthracite  coal,  when  the  pro- 
portion of  coal  is  too  great  for  the  kindling-wood,  the 
heat  generated  by  the  consumption  of  the  wood  fails  to 
ignite  the  coal,  because  such  coal  being  a  comparatively 
good  conductor  of  heat,  the  amount  is  rapidly  distrib- 
uted through  the  mass,  and  hence  the  intensity  at  any 
point  is  insufficient  to  produce  ignition.  But  if  the  pro- 
portion of  coal  be  sufficiently  reduced,  the  consumption 
of  the  same  amount  of  wood  will  produce  ignition. 
The  quantity  of  heat  imparted  to  the  coal  is  the  same  in 
each  case,  but  its  intensity  is  greater  in  the  latter  case. 

In  electric  experiments  there  is  a  great  difference 
noticeable  in  the  amount  of  work  required  to  produce 


8  ELEMENTS  OF  STATIC  ELECTRICITY. 

the  same  electric  intensity  on  different  bodies.  Sealing- 
wax  and  ebonite,  for  instance,  are  quickly  electrified, 
whil-e  brass  is  electrified  slowly.  The  reason  is  analo- 
gous to  that  in  the  illustrations  just  given  :  the  brass 
being  a  good  electric  conductor,  the  electricity  is  in- 
stantly distributed  equally  over  every  part  of  its  sur- 
face, and  hence  the  quantity  at  any  point  being  small, 
the  intensity  is  low.  But  the  sealing-wax  and  ebonite 
being  good  non-conductors,  the  same  quantity  of  elec- 
tricity is  concentrated  on  those  parts  of  the  surface 
brought  into  immediate  contact  with  the  rubber,  instead 
of  being  equally  distributed  over  the  entire  surface  ; 
and  hence  the  intensity  at  those  points  is  proportion- 
ately increased. 

It  will  be  shown  hereafter  that  in  static  electricity 
the  electrification  is  on  the  surface.  Hence,  in  this 
case,  electric  intensity  means  quantity  in  proportion  to 
surface,  whether  it  be  the  entire  surface,  as  on  a  con- 
ductor, or  only  those  parts  to  which  the  electrification 
is  confined,  as  on  a  non-conductor. 

It  must  also  be  understood,  as  will  be  shown  more 
fully  hereafter,  that  the  term  intensity  is  as  applicable 
to  a  diminution  of  electric  energy  at  a  given  point  as 
to  an  increase  ;  in  the  same  sense  as  we  speak  of  intense 
cold,  as  well  as  of  intense  heat. 

STATIC  ELECTRICITY  DEFINED.— The  terms  used  to 
distinguish  different  classes  of  electric  phenomena,  as 
frictional,  static,  galvanic,  chemical,  magneto,  tliermo, 
take  their  origin  from  the  different  methods  by  which 
electricity  is  generated,  and  the  various  conditions  under 
which  its  phenomena  have  been  observed,  and  should 
not  be  understood  as  referring  to  any  difference  in 
the  nature  of  the  electricity  produced. 


STATIC  ELECTRICITY  DEFINED.  9 

The  term  frictional  has  been  used  to  designate  that 
class  of  phenomena  now  under  consideration,  since 
friction  is  one  of  the  principal  agencies  by  which  the 
electricity  is  generated.  But  it  seems  more  appropriate 
to  use  a  term  embracing,  not  merely  one  agency  by 
which  the  electricity  is  generated,  but  also  the  various 
phenomena  produced,  and  distinguishing  these  phenom- 
ena from  those  pertaining  to  electricity  generated  by 
other  methods.  And  since  these  phenomena  refer 
chiefly  to  electricity  when  stationary,  the  term  static, 
from  the  Latin  sto,  to  stand,  has  been  adopted,  to  dis- 
tinguish electricity  observed  under  these  conditions 
from  electricity  observed  chiefly  in  a  state  of  motion. 


CHAPTER  II. 
ELECTRIC   POTENTIAL. 

POTENTIAL. — Potential,  in  the  physical  sense,  is  the 
power  to  accomplish  work.  It  derives  its  specific  name 
from  the  nature  of  the  work,  as  gravity  potential,  heat 
potential,  electric  potential. 

A  pound  weight  raised  to  the  height  of  ten  feet  has 
acquired  ten  foot-pounds  of  gravity  potential,  and  has 
the  power,  if  allowed  to  descend  to  the  same  level,  of 
accomplishing  ten  foot-pounds  of  work,  either  in  rais- 
ing another  weight,  or  setting  machinery  in  motion  by 
which  work  may  be  accomplished. 

A  mass  of  metal  whose  temperature  has  been  raised 
from  zero  to  one  thousand  degrees,  has  acquired  one 
thousand  degrees  of  heat  potential,  and  can  accomplish 
work  to  that  amount  in  cooling  to  zero,  either  by  heat- 
ing another  mass,  or  generating  steam  by  which  machin- 
ery can  be  put  in  motion  and  work  accomplished. 

We  have  seen  that  bodies,  when  electrified,  acquire 
the  power  to  attract  or  repel  other  bodies.  This  power 
is  called  electric  potential. 

Suppose  that  the  electric  energy  of  the  sealing-wax 
in  attracting  the  balanced  rod,  represented  in  Fig.  1, 
Chapter  I.,  were  just  sufficient,  if  expended  without 
loss,  to  move  the  rod  one  foot ;  and,  in  doing  so,  to 
overcome  a  resistance  from  inertia  and  friction  repre- 
sented by  two  ounces  (one-eighth  of  a  pound ) ;  the 


ELECTRIC  POTENTIAL.  11 

electric  potential  of  the  sealing-wax  would  equal  one- 
eighth  of  a  foot-pound. 

If  only  half  this  energy  were  required  to  overcome 
inertia  and  friction,  the  other  half  might  be  expended  in 
lifting  to  a  height  of  one  foot  an  ounce  weight  attached 
to  a  thread  fastened  to  the  end  of  the  rod,  and  passing 
over  a  pulley.  In  which  case  the  work  accomplished 
by  this  half  would  be  represented  by  one-sixteenth  of 
a  foot-pound.  Or  the  weight  might  be  raised,  or  other 
work  to  the  same  amount  accomplished,  by  putting  in 
motion  light  machinery  connected  with  the  rod  by  gear- 
ing at  its  center ;  the  added  friction  being  included  in 
the  ounce  representing  friction  and  inertia. 

Repulsion  would  evidently  produce  the  same  results 
in  this  case  as  attraction. 

To  distinguish  between  electricity  and  electric  poten- 
tial, we  must  bear  in  mind  that  electricity  represents 
the  energy  itself,  while  potential  represents  certain  rela- 
tions between  this  energy  and  matter.  Hence  we  derive 
the  following  definition  : 

Electric  potential  is  the  power  which  a  body  possesses  to 
accomplish  work  by  virtue  of  its  electricity. 

DIFFERENCE  OF  POTENTIAL. — To  accomplish  work 
in  this  way  there  must  first  be  a  difference  of  po- 
tential. 

The  descending  weight  could  not  raise  the  other 
weight  unless  there  was  a  difference  of  level  between 
them.  The  heated  metal  could  not  heat  a  similar  mass 
unless  there  was  a  difference  of  temperature  between 
them.  Neither  could  the  electrified  sealing-wax  attract 
the  rod  unless  there  was  a  difference  of  electric  energy 
between  them.  And  these  phrases,  difference  of  level, 
difference  of  temperature,  difference  of  electric  energy, 


12  ELEMENTS   OF  STATIC  ELECTRICITY. 

are  simply  different  forms  of  expression  for  difference  of 
potential. 

To  produce  this  difference  work  must  first  be  ex- 
pended, and  this  work  is  the  measure  of  the  potential 
acquired. 

The  lifting  of  the  pound  weight  ten  feet  against  the 
force  of  gravity  gave  it  the  ten  foot-pounds  of  gravity 
potential.  The  work  of  heating  the  metal,  whether 
represented  by  combustion,  by  friction,  or  by  concus- 
sion, gave  it  the  one  thousand  degrees  of  heat  potential. 
And  the  rubbing  of  the  sealing-wax  gave  it  the  one- 
sixteenth  of  a  foot-pound  of  electric  potential. 

As  there  is  ordinarily  no  practical  difference  of  elec- 
tric potential  between  different  points  on  the  -earth, 
within  a  limited  area,  its  potential  is  considered  zero, 
and  taken  as  the  base  of  all  measurements  of  electric 
potential. 

The  qualification  of  this  statement,  as  above,  becomes 
necessaiy,  since  there  are  often  great  differences  of  po- 
tential over  widely  separated  areas. 

POSITIVE  AND  NEGATIVE. — Bodies  whose  potential 
is  higher  than  that  of  the  earth  are  said  to  have  positive 
potential,  while  those  whose  potential  is  lower  are  said 
to  have  negative. 

The  potential  of  bodies  is  also  considered  positive  or 
negative  with  reference  to  each  other.  If  a  body  has 
a  higher  potential  than  the  earth,  but  lower  than  that 
of  another  body,  it  is  said  to  be  positive  with  reference 
to  the  earth,  but  negative  with  reference  to  the  other 
body.  In  like  manner  a  body  may  have  negative 
potential  with  reference  to  the  earth,  but  positive  with 
reference  to  another  body  of  lower  potential. 

Hence,  positive  and  negative  are  merely  convenient  rela- 


ELECTRIC  POTENTIAL.  13 

tive  terms  to  designate  different  degrees  of  potential  and 
not  different  kinds  of  electricity. 

The  sign  (  +  )  is  used  to  denote  positive  potential,  and 
( — )  to  denote  negative  potential. 

The  earth's  potential,  then,  is  the  electric  zero,  just 
as  the  freezing  point  is  the  zero  of  temperature  in  the 
centigrade  thermometer,  and  all  uninsulated  bodies  are 
said  to  be  connected  with  the  earth,  and  to  have  zero 
potential  when  not  under  special  influence  from  insu- 
lated, electrified  bodies  in  their  vicinity. 

When  the  electric  potential  of  a  body  is  changed 
from  zero  by  an  increase  of  its  electricity,  it  is  said  to 
be  positively  electrified ;  and  when  its  potential  is 
changed  from  zero  by  a  decrease,  it  is  said  to  be  nega- 
tively electrified. 

ELECTRIC  MOVEMENT. — When  a  difference  of  electric 
potential  exists  between  different  bodies,  or  different 
parts  of  the  same  body,  there  is  a  constant  tendency  to 
equalization. 

A  state  of  equilibrium  seems  to  be  the  natural  condi- 
tion of  bodies,  and  to  produce  difference  of  potential 
requires,  as  we  have  seen,  the  exercise  of  force  in  the 
performance  of  work,  by  which  this  equilibrium  is  dis- 
turbed. 

We  find  in  other  forms  of  energy,  as  gravity  and  heat, 
the  same  tendency  to  equilibrium,  requiring  the  exercise 
of  force  to  overcome  it,  as  in  the  illustrations  already 
given. 

The  restoration  of  equilibrium  is  always  effected  by 
a  transfer  of  energy  from  the  body  having  the  greater 
to  the  one  having  the  less  energy ;  that  is,  from  higher 
to  lower  potential. 

In  the  case  of  gravity  this  transfer  of  energy  carries 


14  ELEMENTS  OF  STATIC  ELECTRICITY. 

the  body  with  it,  as  in  the  descent  of  a  weight  or  the 
movement  of  water  from  a  higher  to  a  lower  level. 
But  in  the  case  of  heat  and  electricity,  the  energy  may 
move  while  the  body  remains  stationary  ;  and  it  may  be 
transferred  from  one  body  to  another,  or  from  one  part 
to  another  of  the  same  body.  Thus  the  mass  of  metal, 
in  the  illustration  given,  transfers  its  heat  energy  to 
another  mass ;  and  in  like  manner,  when  a  metal  rod  is 
heated  at  one  end,  the  heat  moves  to  the  cold  end. 

Gravity  apparently  can  move  only  by  carrying  the 
body  with  it,  while  heat  moves  through  the  body  with- 
out producing  change  of  position  in  its  mass,  like  gravity. 

A  hot  body  transfers  its  heat  to  a  cold  one  in  its 
vicinity,  but  does  not  attract  it ;  while  gravity  produces 
mutual  attraction  between  all  bodies,  but  is  not  trans- 
ferred like  heat  from  one  body  to  another. 

But  in  electrified  bodies  we  have  both  kinds  of  move- 
ment. Like  heat,  electricity  can  move  from  one  body 
to  another,  or  from  one  part  to  another  of  the  same 
body ;  and,  like  gravity,  it  can  carry  the  body  with  it. 

Hence  we  must  distinguish  between  the  movement  of 
electricity  and  the  movement  of  the  electrified  body. 
Electric  movement,  like  heat 'movement,  is  from  higher 
to  lower  potential.  If  one  part  of  a  conductor  be  elec- 
trified, the  electricity  instantly  distributes  itself  over 
every  part.  If  two  insulated  bodies,  free  to  move,  are 
placed  in  each  other's  vicinity,  like  the  pith  balls  of  the 
electroscope,  the  same  tendency  to  equilibrium  is  shown 
by  their  mutual  attraction. 

Though  only  one  ball  be  electrified,  yet  it  is  evident 
that  their  movement  toward  each  other  must  be  mutual, 
and  in  proportion  to  their  mass,  since  action  and  reac- 
tion are  equal :  so  that  while  the  movement  of  the  elec- 


ELECTRIC  POTENTIAL.  15 

tricity  is  from  the  electrified  to  the  non-electrified  ball, 
that  is,  from  higher  to  lower  potential,  the  movement 
of  the  balls  is  mutual. 

It  will  also  be  noticed  that  the  movement  of  the  non- 
electrified  ball  is  opposite  to  that  of  the  electricity. 
Hence,  while  electricity  moves  from  higher  to  lower 
potential,  bodies  under  its  influence  may  move  in  either 
direction. 

We  have  seen  that  when  the  two  balls  come  into 
contact  there  is  a  transfer  of  electricity  from  the  elec- 
trified to  the  non-electrified  ball ;  equilibrium  is  estab- 
lished, and  mutual  repulsion  follows,  not  only  between 
the  balls,  but  also  between  them  and  the  electrified 
sealing-wax. 

So  long  as  a  difference  of  potential  exists  there  is 
mutual  attraction ;  but  when  equilibrium  is  established 
there  is  mutual  repulsion.  The  same  results  may  be 
produced  by  numerous  similar  experiments,  in  which 
different  substances  and  different  methods  may  be 
employed.  Hence  we  deduce  the  following  important 
principle : 

Electrified  bodies  at  different  potentials  attract,  while 
those  at  the  same  potential  repel  each  other. 

There  can  be  no  repulsion  unless  there  is  a  difference 
of  potential  between  the  electrified  bodies  and  their 
surroundings.  For  if  the  surrounding  bodies  \vere  at 
the  same  potential  as  the  electrified  bodies,  the  repul- 
sion would  be  neutralized  by  their  reaction.  Hence 
bodies  at  zero  potential  can  show  no  repulsion.  But  in 
all  cases  of  electrification  there  is  a  difference  of  poten- 
tial created  in  the  body,  either  above  or  below  the  origi- 
nal zero. 

Indeed,  attraction    may  account    for    the    apparent 


16 


ELEMENTS  OF  STATIC  ELECTRICITY. 


mutual  repulsion  of  bodies  at  the  same  potential,  since 
this  difference  of  potential  between  the  electrified  bod- 
ies and  their  surroundings  must  produce  attraction  and 
tend  to  separate  them. 

But  such  outward  attraction  would  not  disprove  the 
existence  of  repulsion,  though  it  might  account  for 
some  of  its  phenomena. 

THE  GOLD  LEAF  ELECTROSCOPE  — As  our  investi- 
gations now  require  a  more  sensitive  instrument  than 
any  which  has  yet  been  described,  we  here  introduce 
the  gold  leaf  electroscope. 


Fig.  4— Gold  Leaf  Electroscopes. 

The  style  represented  at  A,  Fig.  4,  is  convenient,  and 
easily  constructed.  It  consists  of  a  half-gallon  tincture 
bottle,  fitted  with  an  ebonite  stopper,  through  the  center 
of  which  passes  a  small  brass  rod  about  five  inches  long, 
which  terminates  about  three-fourths  of  an  inch  above 
the  stopper  in  a  brass  disc  about  two  inches  in  diame- 
ter, having  a  round  rim  about  three-sixteenths  of  an 


ELECTRIC  POTENTIAL.  17 

inch  ill  diameter  projecting  from  its  lower  surface,  as 
shown  in  the  enlarged  section  at  D. 

To  the  lower  end  of  the  rod  is  attached  a  thin  cross- 
bar, about  five-eighths  of  an  inch  long,  which  will  pass 
easily  through  the  neck  of  the  bottle.  And  from  this 
cross-bar  are  suspended  two  strips  of  imitation  gold  leaf, 
each  five-eighths  of  an  inch  wide  by  2J  inches  long.  A 
small  hole  is  drilled  near  the  edge  of  the  disc  for  con- 
venience in  attaching  wires. 

The  leaves  in  this  instrument  lie  close  together,  and, 
consequently,  must  always  be  electrified  at  the  same 
potential;  but  in  some  experiments  it  is  desirable  to 
electrify  them  separately,  and  for  this  purpose  .a  bottle 
with  a  wide  neck  is  used,  which  will  admit  an  ebonite 
stopper  through  which  two  rods  can  be  inserted  about 
an  inch  apart,  and  from  the  cross-bar  of  each  a  single 
leaf  is  suspended,  the  surfaces  being  parallel  to  each 
other.  This  style  is  represented  at  #,  Fig.  4.  The  rods 
can  terminate  above  in  balls,  or  be  bent  outward  and 
terminate  in  discs. 

Electroscopes  may  be  constructed  with  thin  metal 
discs,  attached  to  the  glass  opposite  the  leaves ;  strips 
of  the  same  material  extending  down  and  connecting 
with  the  earth.  Brass  rods  surmounted  with  balls  are 
often  used  in  the  same  way,  as  represented  at  (7,  Fig.  4 ; 
in  which  case  a  glass  shade  resting  on  a  wooden  base  is 
more  convenient  than  the  bottle  form. 

The  object  in  either  case  is  to  have  conductors  at  zero 
potential  near  the  leaves,  which  renders  them  niore 
sensitive,  and  discharges  them  in  case  of  too  great 
divergence  ;  thus  preventing  their  adhesion  to  the  glass, 
which  is  often  troublesome.  Annoyance  from  the  latter 
cause  is  also  obviated  by  using  a  bottle  of  globular  form, 


18  ELEMENTS  OF  STATIC  ELECTRICITY. 

the  sides  of  which  are  too  remote  to  be  touched  by  the 
leaves. 

A  brass  cap,  covering  the  glass  above,  as  shown  at  (7, 
is  also  used  to  screen  the  leaves  from  external  electric 
influence,  and  wire  screens  are  likewise  used  for  the 
same  purpose. 

The  use  of  the  bottle,  or  glass  shade,  is  to  protect  the 
leaves  from  currents  of  air  which  would  destroy  them. 
And  the  ebonite  stopper  is  for  better  insulation,  since 
the  glass  generally  used  for  bottles  and  shades  is  of 
inferior  insulating  quality.  The  disc,  or  ball,  and  con- 
necting-rod are  for  convenience  in  electrifying  the 
leaves,  which  are  the  efficient  part  of  the  instrument. 

The  following  experiment  will  illustrate  its  use  :— 
Let  the  electrified  sealing-wax  touch  the  disc  of  electro- 
scope A  ;  electricity  is  instantly  transferred  to  the  disc, 
rod,  and  leaves,  which  are  all  good  conductors,  and  the 
leaves,  being  free  to  move,  and  at  the  same  potential, 
repel  each  other,  and  diverge. 

If  the  disc  now  be  touched  with  the  finger,  the  elec- 
tricity escapes  to  the  earth,  and  the  leaves,  being  reduced 
to  zero,  converge. 

The  sensitiveness  of  this  instrument  is  so  great  that  a 
chip  of  dry  wood,  less  than  a  grain  in  weight,  electrified 
in  cutting,  and  dropped  on  the  disc,  produces  divergence 
of  the  leaves.  A  wooden  pen-holder,  electrified  by  strik- 
ing it  lightly  on  the  table,  produces  the  same  effect. 

Hence,  care  must  be  observed  to  prevent  the  leaves 
from  being  torn  by  sudden,  spasmodic  movements,  Avhich 
are  liable  to  occur  when  experimenting  with  highly  elec- 
trified bodies  in  their  vicinity. 

MUTUAL  EFFECTS  OF  FRICTION. — Thus  far  we  have 
considered  only  the  effect  produced  on  the  sealing-wax, 


ELECTRIC  POTENTIAL.  19 

glass,  or  other  substance  electrified  by  friction,  without 
reference  to  the  effect  on  the  substance  by  which  it  was 
rubbed.  But  since  action  and  reaction  are  equal,  it  is 
evident  that  these  two  effects  must,  in  some  way,  equal 
each  other ;  that  electricity,  or  its  equivalent  in  some 
other  form  of  energy,  must  be  produced  on  the  rubber 
as  well  as  on  the  substance  rubbed. 

To  test  this,  let  a  piece  of  flannel,  after  being  used  to 
rub  a  stick  of  sealing-wax,  touch  the  disc  of  electroscope 
-4,  Fig.  4,  and  the  leaves  will  instantly  diverge,  showing 
that  the  flannel  has  been  electrified. 

Substitute  silk,  fur,  or  any  other  substance  used  as  a 
rubber,  and  the  same  result  will  follow.  Let  the  various 
substances  rubbed  be  also  tested,  and  it  will  be  found 
that  electrification  has  been  produced  on  both  rubber 
and  substance  rubbed,  at  the  same  time,  by  the  same 
process. 

Now  let  a  rubber,  about  the  same  size  as  the  sealing- 
wax,  be  prepared,  by  wrapping  a  strip  of  wood  in  flannel 
and  insulating  one  end  with  a  piece  of  india-rubber 
tube. 

Holding  this  rubber  by  the  insulated  end,  let  the 
sealing-wax  be  rubbed  with  it ;  and,  keeping  both  still 
in  contact,  lay  them  carefully  on  the  disc  of  the  electro- 
scope, so  that  both  shall  touch  it  at  the  same  instant, 
and  no  divergence  of  the  leaves  will  occur.  Now  lift 
off  the  sealing-wax  and  they  instantty  diverge ;  replace 
it  and  they  converge.  Lift  off  the  rubber  and  they 
diverge,  replace  it  and  they  converge  again. 

Let  the  experiment  be  made  with  any  other  two  sub- 
stances used  to  generate  electricity  by  friction,  as  silk 
and  glass,  ebonite  and  fur,  and  similar  results  will  be 
obtained. 


20  ELEMENTS  OF  STATIC  ELECTRICITY. 

It  will  also  be  noticed  that  the  approach  of  either 
electrified  body  while  the  other  lies  on  the  disc  causes 
the  leaves  to  converge,  while  its  withdrawal  produces 
divergence. 

There  is  often  a  slight  divergence  of  the  leaves  when 
both  bodies  are  in  contact  on  the  disc,  due  to  the  diffi- 
culty of  producing  perfect  adjustment  of  contact,  and 
also  to  the  fact  that  the  electric  condition  of  one  body 
may  change  more  rapidly  than  that  of  the  other,  from 
imperfect  insulation  or  other  cause. 

The  amount  of  divergence  is  also  liable  to  vary,  the 
removal  of  one  body  producing  greater  divergence  than 
the  removal  of  the  other.  This  difference  is  also  easily 
accounted  for  by  difference  of  mass,  of  conductivity,  or 
other  cause. 

Hence  we  deduce  the  following  rule :  When  electricity 
is  generated  on  two  bodies  by  their  mutual  friction,  the  elec- 
tricity of  each  is  neutralized  by  the  presence  of  the  other. 

The  effect  of  the  mutual  friction  of  the  two  bodies  is 
to  create  a  difference  of  potential  by  the  transfer  of 
electric  energy  from  one  to  the  other.  As  one  gains 
what  the  other  loses,  the  amount  of  energy  on  the  two 
is  not  changed  so  long  as  they  remain  in  contact,  and 
hence  the  potential  of  the  electroscope  is  not  disturbed. 

But  let  one  of  the  bodies  be  removed  ;  suppose  it  to 
be  the  one  to  which  energy  has  been  transferred,  the 
potential  of  the  remaining  body  being  negative,  there  is 
instantly  a  transfer  of  energy  to  it  from  the  disc  and 
leaves,  which  thus  become  negative  also. 

The  leaves,  being  both  at  the  same  potential,  diverge 
by  mutual  repulsion  ;  and  that  potential  being  less  than 
zero,  the  divergence  is  increased  by  attraction  from  the 
higher  potential  of  the  glass  and  surrounding  objects. 


ELECTRIC  POTENTIAL.  21 

Replacing  this  body,  let  the  one  from  which  energy 
has  been  transferred  be  removed ;  the  potential  of  the 
remaining  body  being  positive,  there  is  instantly  a 
transfer  of  energy  from  it  to  the  disc  and  leaves,  making 
them  positive  also.  Hence  the  leaves  diverge  as  before, 
from  mutual  repulsion,  and  the  divergence  is  increased 
by  attraction  from  the  lower  potential  of  the  glass  and 
surrounding  objects. 

From  this  it  will  be  seen  that  the  effect  on  the  electro- 
scope is  the  same  whether  the  potential  of  the  electrified 
body  be  positive  or  negative.  In  either  case  there  is 
mutual  repulsion  between-  the  leaves,  from  their  being 
at  the  same  potential ;  and  mutual  attraction  between 
them  and  surrounding  objects,  caused  by  difference  of 
potential. 

The  indications  of  the  electroscope  furnish  no  means 
of  distinguishing  between  positive  and  negative  poten- 
tial, being  the  same  for  both.  And  as  this  is  true  of 
most  of  the  phenomena  pertaining  to  these  two  states, 
it  is  difficult,  in  static  electricity  especially,  to  determine 
which  phenomena  are  positive  and  which  negative. 

There  is  no  such  well-marked  distinction  between 
them  as  between  the  positive  and  negative  states  known 
as  heat  and  cold  ;  neither  can  we  observe  electric  move- 
ment as  we  can  heat  movement;  since  heat  moves 
slowly,  while  electricity  moves  with  inconceivable  ra- 
pidity. 

But  if  we  can  show  cause  for  an  accumulation  of  elec- 
tric energy  at  one  point  and  for  its  absence  at  another, 
and  show  effects  following  such  difference  of  energy,  we 
then  have  proof  of  the  positive  and  negative  potential 
of  the  different  points,  which  may  be  accepted  as 
reliable. 


22  ELEMENTS  OF  STATIC  ELECTRICITY. 

Such  proof  will  be  furnished  hereafter,  and  the  further 
consideration  of  this  question  must  be  deferred  till  the 
examination  of  other  phenomena  shall  enable  the  stu- 
dent to  comprehend  such  proof. 

CHARGE  DEFINED. — The  term  charge  is  used  to  ex- 
press the  condition  of  an  electrified  body  when  its  poten- 
tial is  above  or  below  zero.  If  its  potential  has  been 
raised  above  zero  by  receiving  electricity,  it  is  said  to  be 
positively  charged  ;  but  if  its  potential  has  been  reduced 
below  zero  by  the  removal  of  electricity,  it  is  said  to  be 
negatively  charged. 

Hence  we  speak  of  a  high  negative  charge  in  the  same 
sense  as  we  speak  of  intense  cold,  meaning  an  intensity 
of  the  negative  condition  caused  by  the  absence  of  heat. 


CHAPTER  III. 
THE  NATURE  OF  ELECTRICITY. 

THE  CONSERVATION  OF  ENERGY. — A  clear  under- 
standing of  that  great  doctrine  of  modern  science, 
known  as  the  conservation  of  energy,  lies  at  the  founda- 
tion of  a  correct  knowledge  of  electricity  and  electric 
phenomena.  Hence  a  brief  examination  of  its  prin- 
ciples will  not  be  out  of  place  here. 

Energy  is  a  universal  property  of  matter.  It  is  the 
principle  of  life  and  movement  in  matter  in  distinc- 
tion from  matter  itself,  inseparably  connected  with 
matter  and  yet  distinct  from  it :  heat  as  distinct  from 
the  heated  body ;  electricity  as  distinct  from  the  elec- 
trified body ;  life  as  distinct  from  the  living  body. 

Like  matter,  it  manifests  itself  in  various  forms, 
as  gravity,  cohesion,  chemical  affinity,  light,  heat,  elec- 
tricity. Like  matter,  its  quantity  in  the  universe  is 
fixed  and  definite,  and  cannot  be  increased  or  dimin- 
ished. And  hence,  like  matter,  it  is  indestructible. 

It  may  be  transmuted  from  one  form  into  another, 
but  in  the  transmutation  there  is  no  loss.  One  form 
may  re-appear  in  many  forms,  or  the  many  be  reduced 
to  the  one. 

In  our  experiments,  muscular  energy  has  been  ex- 
pended to  produce  electric  energy ;  but  the  energy  pro- 
duced must  equal  that  which  produced  it,  if  the  doc- 
trine of  the  conservation  of  energy  is  true.  And  since 


24  ELEMENTS  OF  STATIC  ELECTRICITY. 

it  is  evident  that  only  a  very  small  part  of  the  mus- 
cular energy  expended  would  be  required  to  move 
the  pith  balls,  the  balanced  rod,  or  the  gold  leaves,  the 
remainder  must  be  accounted  for. 

This  is  easily  done  when  we  consider,  first,  that 
the  electric  energy  was  equally  divided  between  the 
rubber  and  the  substance  rubbed;  secondly,  that  only 
a  small  part  of  the  electric  energy  was  used ;  that  the 
electricity  generated  was  sufficient  for  the  performance 
of  the  same  work  many  times  in  succession,  cither 
with  the  rubber  or  substance  rubbed;  and  that  a 
number  of  pith  balls,  placed  on  all  sides  of  the 
electrified  body,  might  have  been  subjected  to  its 
influence.  Thirdly,  we  must  consider  the  amount  of 
electricity  lost  from  contact  with  the  surrounding  air; 
and,  lastly,  that  the  amount  of  heat  energy  gener- 
ated by  the  friction  was  probably  equal  to  the  electric 
energy. 

If  the  expended  energy  had  been  produced  by  a 
descending  weight,  which  should  cause  a  glass  or 
ebonite  cylinder  to  revolve  in  contact  with  a  rubber, 
and  the  sum  total  of  the  heat  and  electricity  had 
been  recovered  in  the  form  of  work  which  could  be  es- 
timated, it  would  be  found  so  nearly  equal  to  the 
number  of  foot-pounds  expended  by  the  descending 
weight,  that  whatever  difference  existed  could  easily 
be  accounted  for  by  the  friction  of  the  machinery  and 
other  causes. 

Experiments  of  this  kind  have  been  actually  per- 
formed, and  the  results  verify  the  above  conclusion. 
Similar  experiments  have  also  been  made  with  other 
forms  of  energy,  and  like  results  obtained ;  so  that 
the  principles  of  the  conservation  of  energy  are  now 


THE  NATURE  OF  ELECTRICITY.  25 

well  established,  and  universally  recognized  in  all 
practical  work. 

Another  illustration  may  render  the  subject  more 
clear.  A  pound  weight  raised  to  a  height  of  20  feet 
has  acquired  20  foot-pounds  of  energy ;  and,  in  de- 
scending to  its  former  level,  can  accomplish  20  foot- 
pounds of  work;  as  in  raising  to  the  same  height 
another  weight  of  nearly  equal  mass.  But  if  stopped 
in  its  descent  at  a  level  of  10  feet,  it  has  expended 
only  10  foot-pounds  of  energy,  and  has  still  a  reserve  of 
10  more. 

It  is  evident,  that  to  raise  the  weight  in  the  first  place 
required  the  expenditure  of  20  foot-pounds  of  energy ; 
and  that  though  this  energy  was  consumed  in  the  pro- 
cess, and  had  disappeared,  it  was  not  lost,  but  merely 
stored  up,  ready  to  be  expended,  either  at  once,  by 
the  weight  descending  the  entire  distance,  or  in  detail, 
as  when  stopped  half  way  or  at  any  other  point.  If  it 
had  descended  but  one  foot,  it  would  still  have  a  re- 
serve of  19  foot-pounds  of  energy. 

It  will  be  noticed  that  the  second  weight,  raised  by 
the  descent  of  the  first,  is  required  to  be  of  less 
magnitude,  since  part  of  the  energy  must  be  expended 
in  overcoming  friction  and  inertia.  For  if  it  were  of 
equal  magnitude,  the  force  expended  would  exceed  the 
force  stored  up;  since  it  must  perform  not  only  the 
same  work,  but  the  added  amount  for  friction  and 
inertia;  in  which  case  it  would  be  possible  to  create 
force,  and  the  doctrine  of  the  conservation  of  energy 
would  cease  to  be  true. 

It  is  immaterial  whether  the  descent  of  the  one  pound 
raises  a  small  weight  to  the  height  of  20  feet,  or  a 
laree  weight  to  the  height  of  one  foot ;  which  it  can  be 


26  ELEMENTS  OF  STATIC  ELECTRICITY. 

made  to  do  by  a  system  of  ropes  and  pulleys.  The 
20  foot-pounds  of  energy  expended  must  exactly 
equal  the  20  foot-pounds  stored  up;  and  the  height 
through  which  the  large  weight  is  raised  is  to  that 
through  which  the  small  weight  descends  in  the  in- 
verse ratio  of  the  mass  of  each. 

In  all  electric  work,  of  whatever  nature,  the  same 
principle  will  be  found  to  hold  true ;  gravity  potential 
in  this  case  representing  electric  potential  in  electric 
work.  Mechanical  work  may  re-appear  as  electric  work, 
or  electric  as  mechanical  work;  the  energy  produced 
being  always,  in  some  form,  equal  to  the  energy 
expended. 

PIEAT,  LIGHT,  AND  ELECTRICITY  COMPARED. — The 
weight  of  evidence  goes  to  show  that  electricity,  like 
heat  and  light,  belongs  to  that  kind  of  energy  known 
as  molecular  ;  and  whatever  is  known  as  to  one  kind  of 
energy  may,  by  analogy,  be  inferred  as  to  other  kinds 
of  the  same  class,  with  such  modifications  as  distin- 
guish different  species  of  the  same  genus. 

There  is  ample  proof  that  heat  is  a  mode  of  molec- 
ular motion.  Not  that  the  heat  produces  the  motion,  or 
the  motion  the  heat,  but  that  it  is  motion  ;  that  the 
molecules  of  matter  being  thrown  into  a  certain  kind 
of  motion,  the  result  is  the  sensation  known  as  heat. 

According  to  the  universally  accepted  theory  of 
light,  it  is  another  species  of  motion  of  the  same 
kind ;  and  there  are  indications  that  light  and  elec- 
tricity are  identical.  But,  if  not  identical,  we  may 
at  least  assume  that  they  are  closely  allied  to  each 
other. 

We  find  also  that  the  same  causes,  acting  at  the 
same  time,  on  the  same  bodies,  and  under  the  same 


TIIE  NATURE  OF  ELECTRICITY.  27 

circumstances,  produce  both  heat  and  electricity,  in 
numerous  instances;  in  others,  equally  numerous,  both 
heat  and  light,  and  in  others,  heat,  light,  and  elec- 
tricity. 

The  simultaneous  production  of  heat  and  electric- 
ity is  seen  in  the  examples  already  given  of  bodies 
electrified  by  friction,  of  which  heat  is  also  a  neces- 
sary result. 

Another  prominent  instance  is  the  action  of  the 
electric  generators  known  as  dynamos;  in  which  the 
evolution  of  heat  is  such  that  special  provision  for  cool- 
ing has  to  be  made,  to  prevent  injury.  Here  mechani- 
cal action  is  the  agent. 

In  the  galvanic  battery  we  have  a  well-known  in- 
stance of  the  production  of  heat  and  electricity  by 
chemical  action ;  as  a  certain  amount  of  heat,  more  or 
less  perceptible,  is  always  a  result. 

Instances  of  the  simultaneous  production  of  heat 
and  light  are  numerous  and  well  known,  as  the  heat- 
ing of  an  iron  rod,  which  becomes  luminous  when  the 
temperature  rises  to  a  certain  degree ;  whether  it  be 
heated  by  friction,  as  of  a  shaft  and  journal,  or  by  the 
chemical  action  of  a  furnace. 

The  dynamo  and  galvanic  battery  have  been  re- 
ferred to  as  producing  both  heat  and  electricity. 
When  a  current  of  this  electricity,  of  sufficient  in- 
tensity, is  passed  through  a  conductor  of  high  resist- 
ance, as  a  fine  platinum  wire,  or  a  carbon  filament,  they 
become  luminous  by  incandescence  ;  and  when  passed 
through  two  sticks  of  carbon  slightly  separated,  we  have 
light  of  great  intensity  ;  and  there  is,  in  both  instances, 
the  evolution  of  intense  heat.  These  are  perhaps  the 
most  striking  examples  which  can  be  given  of  the  sim- 


28  ELEMENTS  OF  STATIC  ELECTRICITY. 

ultaneous  evolution  of  heat,  light,  and  electricity  from 
the  same  causes. 

In  the  thermo-electric  battery  we  have  an  example 
of  the  direct  production  of  electricity  by  heat. 

POLARIZED  LIGHT  AND  ELECTKICLTY. — Experiments 
with  polarized  light,  made  by  Faraday  and  others,  fur- 
nish strong  evidence  of  the  close  alliance,  if  not  actual 
identity,  of  light  and  electricity. 

It  is  known  that  light,  from  certain  peculiarities  of 
reflection  and  transmission,  undergoes  a  change,  so 
that  when  subsequently  transmitted,  its  action  is  dif- 
ferent from  that  of  the  original  transmission,  and  this 
change  has  been  termed  polarization. 

Let  a  plate  of  tourmaline  be  so  placed  that  a  ray  of 
light  falling  on  it  shall  be  transmitted  at  right  angles 
to  a  particular  direction  within  the  crystal,  known  as 
its  optical  axis.  Then  let  another  tourmaline  plate  be 
so  placed  with  reference  to  this  one  that  their  optical 
axes  are  parallel,  and  that  the  ray  shall  pass  through 
both  and  form  a  bright  spot  on  a  screen  beyond. 

Now  let  either  plate  be  turned,  so  that  their  optical' 
axes  are  no  longer  parallel  to  each  other,  but  still  at 
right  angles  to  the  ray ;  the  bright  spot  on  the  screen 
will  fade  as  the  angle  increases,  till  at  90  degrees  it  will 
disappear.  Continuing  the  rotation,  it  will  re-appear, 
increasing  in  brightness,  till,  at  180  degrees,  it  is  en- 
tirely restored ;  then  fading  again  till  extinguished 
at  the  end  of  the  third  quadrant,  and  again  increasing 
in  brightness  till  restored  at  the  end  of  the  fourth 
quadrant,  or  original  position. 

This  alternation  of  brightness  and  extinction  de- 
pends on  the  relative  angular  position  of  the  plates. 
Let  either  of  them  be  turned  in  either  direction,  and 


THE  NATURE  OF  ELECTRICITY.  29 

the  same  result  follows ;  but  when  both  are  turned  in 
the  same  direction,  there  is  no  change  in  the  brightness, 
and  no  extinction  of  the  light.  Substitute  one  for  the 
other,  and  the  same  results  are  obtained. 

It  is  evident,  then,  that  the  light  in  passing  through  the 
first  plate  has  undergone  a  change  which  affects  its  trans- 
mission through  the  second,  in  any  position  except  when 
the  optical  axes  of  both  are  parallel;  extinguishing  it 
entirely  when  they  are  at  right  angles  to  each  other. 

It  is  also  found  that  this  effect,  termed  polarization, 
occurs  to  light  transmitted  through  or  reflected  from  any 
transparent  medium,  as  glass,  selenite,  Iceland  spar, 
and  various  liquids.  Polished  metals  also  produce  the 
same  effect  on  reflected  light.  And  this  reflection  or 
transmission  takes  place  at  a  certain  angle,  known  as 
the  polarizing  angle,  which  varies  in  each  substance 
by  a  certain  definite  amount. 

Now  let  the  ray  be  transmitted  through,  or  reflected 
from,  a  small  piece  of  glass  of  suitable  size  or  shape, 
placed  at  the  proper  polarizing  angle,  and  let  the  plates 
be  turned  till  their  optical  axes  are  at  right  angles,  so 
as  to  produce  extinction ;  then  let  the  glass  be  sub- 
jected to  a  powerful  electric  strain  and  the  extinguished 
light  will  re-appear,  continue  during  the  electric  action, 
and  disappear  when  it  ceases ;  which  shows  that  this 
electric  action  has  counteracted  the  effects  of  polar- 
ization. 

Similar  experiments  with  various  substances,  too 
numerous  to  detail  here,  show  similar  results. 

Without  anticipating  another  branch  of  our  subject, 
it  may  be  stated  here,  that  electricity  and  magnetism 
are  so  closely  allied  that  whatever  affects  one  must 
have  some  important  relation  to  the  other. 


30  ELEMENTS  OF  STATIC  ELECTRICITY. 

Recent  experiments  have- demonstrated  that  unusual 
disturbances  in  the  variation  of  the  magnetic  needle 
are  coincident  with  unusual  disturbances  in  the  sun,  in 
connection  with  the  phenomena  known  as  sun  spots ; 
and  that  the  telegraph  and  telephone  are,  at  such 
times,  seriously  disturbed  by  what  are  known,  techni- 
cally, as  "electric  storms";  that  is,  unusual  disturb- 
ances in  the  earth's  electricity  shown  in  the  phenomena 
known  as  earth  currents. 

It  has  also  been  found  that  these  solar  disturbances 
are  periodic;  and  that  these  periods,  for  the  last  hundred 
and  fifty  years,  correspond  almost  exactly  with  the 
periods  of  unusual  variation  of  the  magnetic  needle. 

Since  the  sun  is  our  chief  source  of  light  and  heat, 
since  they  are,  in  fact,  the  result  of  the  constant  dis- 
turbance of  the  elements  of  that  body;  and  since,  when 
this  disturbance  assumes  an  unusual  phase,  there  is,  co- 
incident with  it,  an  unusual  disturbance  in  the  earth's 
electricity,  it  must  be  accepted  as  strong  proof  of  a 
common  origin  of  heat,  light,  and  electricity. 

We  have  heat,  light,  and  electricity  derived  from 
friction,  from  chemical  action,  from  magnetic  action, 
and  from  the  sun.  We  have  heat  producing  electric- 
ity, and  electricity  producing  heat  and  light ;  and  we 
have  electricity  neutralizing  the  polarizing  effect  of 
light.  The  evidence  of  identity  then  becomes  cumu- 
lative, while  that  of  close  alliance  amounts  to  demon- 
stration. Hence  we  may  infer  certain  facts  in  regard 
to  the  nature  of  electricity  from  what  we  know  of 
similar  facts  in  regard  to  the  nature  of  light  and  heat. 
And  we  are  also  warranted  in  the  conclusion,  that  a 
well  supported  theory  of  light  or  heat  requires  but 
little  modification  to  adapt  it  to  electricity. 


THE  NATURE  OF  ELECTRICITY.  31 

l'*: 
This  much  we  certainly  know,  that  they  all  are  forms 

of  energy  and  that  they  radiate  from  the  centers  where 
they  are  generated.  Hence  the  term  radiant  energy 
is  equally  applicable  to  each. 

THE  WAVE  THEORY. — It  has  been  assumed  that  a 
subtle  medium,  termed  ether,  pervades  all  space  ;  that 
it  is  so  attenuated  that  it  can  insinuate  itself  between 
the  grosser  molecules  of  material  bodies ;  so  that  solids 
of  the  finest  and  closest  texture,  as  well  as  liquids  and 
gases,  are  pervaded  by  it;  and  that  light,  and  probably 
electricity,  are  due  to  waves  or  undulations  of  this 
ether.  The  evidence  of  the  existence  of  such  a  medium 
is  almost  wholly  negative,  and,  like  all  negative  evi- 
dence, unsatisfactory.  The  assumption  presupposes  the 
necessity  of  its  existence. 

It  has  been  stated  that  energy  is  a  universal  property 
of  matter,  and  the  converse  may  be  accepted,  that 
energy  cannot  exist  without  matter.  And  since  light, 
coming  from  the  sun,  must  traverse  the  in'terplane- 
tary  spaces,  there  must  be  matter  there ;  else  we  shall 
be  compelled  to  admit  that  energy  can  exist  without 
matter,  which  is  contrary  to  all  our  experience. 

The  earth  is  surrounded  by  an  atmosphere,  to  the 
limits  of  which  we  cannot  penetrate.  In  1822,  Dr. 
Wollaston  made  a  careful  mathematical  calculation,  as 
the  result  of  which  he  claimed  to  have  demonstrated 
that  the  earth's  atmosphere  must  have  limits,  beyond 
which  it  cannot  exist.  And  this  apparent  demonstra- 
tion was  accepted  as  authority,  and  remained  unchal- 
lenged for  half  a  century.  Meantime  the  wave  theory 
of  light  became  prominent,  and  a  medium  being  one  of 
its  fundamental  principles,  the  existence  of  the  ether 
was  assumed,  and  is  now  generally  accepted. 


32  ELEMENTS  OF  STATIC  ELECTRICITY. 

But  the  researches  of  modern  science  have  swept 
away  many  of  the  errors  of  the  past,  and  it  is  not  im- 
possible that  Dr.  Wollaston's  demonstration  may  share 
the  same  fate.  Many  eminent  scientists,  who  have 
made  experimental  investigations  on  the  subject,  hold 
that  the  expansibility  of  the  earth's  atmosphere  is 
unlimited ;  among  whom  may  be  cited  Grove,  Gassiot, 
Geissler,  and  Dr.  Andrews.  And  W.  M.  Williams, 
in  his  work,  "  The  Fuel  of  the  Sun,"  claims  to  have 
discovered  a  serious  error  in  Dr.  Wollaston's  calcula- 
tions, which  vitiates  his  conclusion. 

The  assumption  of  these  writers  is  that  an  atmos- 
phere, the  same  as  that  of  our  earth,  pervades  all 
space  ;  that  in  the  interplanetary  spaces  it  becomes  ex- 
ceedingly attenuated  ;  and  that  each  of  the  heavenly 
bodies  attracts  and  surrounds  itself  with  a  portion  of 
it ;  the  extent  and  density  of  which  is  in  proportion  to 
the  mass  of  the  body. 

The  high  degree  of  vacuum  which  can  now  be  attained 
by  improvements  in  the  air-pump,  seems  to  demonstrate, 
that  while  electricity  will  pass  more  freely  through  rare- 
fied air,  on  account  of  the  reduced  resistance,  than 
through  air  of  ordinary  density,  it  must  still  have  a 
medium  in  which  to  travel ;  and  that  its  passage  through 
an  absolute  vacuum,  or  space  devoid  of  any  known  ma- 
terial substance,  is  highly  improbable.  But  as  the  best 
attainable  vacuum  is  still  only  an  approximation  to  an 
absolute  vacuum,  the  full  demonstration  of  this  point 
has  not  yet  been  reached. 

The  existence  then  of  some  elastic  medium,  by  which 
the  two  forms  of  radiant  energy,  known  as  light  and 
heat,  can  traverse  the  interplanetary  spaces,  is  not 
questioned.  Nor  does  the  theory  of  the  unlimited  ex- 


THE  NATURE  OF  ELECTRICITY.  33 

pansion  of  our  atmosphere  conflict  at  all  with  the 
theory  of  the  universal  existence  of  ether,  since 
the  theory  of  ether  is  that  it  permeates  all  material 
substances. 

The  wave  theory  assumes  that  radiant  energy  is 
transmitted  by  the  undulations  of  some  medium ;  that 
an  impulse  originating  at  any  center  of  energy,  as  the 
sun,  produces  a  wave  which  traverses  this  medium  with 
inconceivable  velocity,  till  it  reaches  some  distant 
point,  as  the  earth  ;  and  that  the  constancy  of  such  im- 
pulses at  every  point  on  the  sun  gives  rise  to  the  phe- 
nomena of  solar  light,  heat,  and  electricity. 

In  like  manner  we  may  assume  any  other  center  of 
energy,  as  a  red-hot  metal  ball,  radiating  light  and 
heat;  a  stick  of  ebonite,  excited  by  friction,  radiating 
electricity. 

It  is  also  assumed  that  the  impulses  radiate  in  straight 
lines,  while  the  undulations  occur  at  right  angles  to 
those  lines. 

To  illustrate  : —  Drop  a  pebble  on  a  smooth  sheet  of 
water ;  the  impulse  creates  waves  which  radiate  out- 
ward in  widening  circles.  The  pebble  has  depressed 
the  water  at  the  point  where  it  struck,  forcing  the  ad- 
jacent water  outward,  and  causing  it  to  rise  above  the 
general  level ;  then  the  downward  impulse  of  this 
wave,  cinking  under  the  force  of  gravity,  raises  the 
original  center,  and  also  produces,  by  its  outward  im- 
pulse, another  wave  beyond,  as  it  descends  by  its  inertia 
below  the  general  level. 

As  the  water  oscillates  vertically  above  and  below  the 
level,  each  successive  impulse  produces  a  new  wave, 
while  the  same  process  goes  on  in  the  outward  waves, 
creating  new  waves  beyond,  in  ever  widening  circles, 


84  ELEMENTS  OF  STATIC  ELECTRICITY. 

till  the  force  of  the  original  impulse  has  been  exhausted, 
and  the  water  returns  to  its  original  level. 

Now  it  will  be  perceived  that  there  is  no  transfer  of 
the  water  from  the  center  outward  beyond  the  length  of 
the  first  wave.  Part  of  the  water  forced  outward  by  the 
original  impulse  flows  back  again,  while  another  part 
flows  outward,  producing  a  new  wave.  The  water  is  then 
under  the  influence  of  two  forces,  one  horizontal,  the 
other  vertical,  acting  at  right  angles  to  each  other;  the 
horizontal  producing  the  wave  length,  that  is,  the 
distance  from  crest  to  crest,  or  from  hollow  to  hollow, 
while  the  vertical  produces  the  height,  that  is,  the 
vertical  distance  from  hollow  to  crest,  or  from  crest  to 
hollow. 

In  a  similar  way,  it  is  supposed,  occur  the  undula- 
tions of  the  assumed  elastic  medium,  with  this  excep- 
tion, that  the  waves  on  the  water  occur  in  the  same 
horizontal  plane,  radiating  outward  in  concentric  cir- 
cles, while  those  in  the  elastic  medium  occur  in  any 
direction  in  which  they  are  free  to  move ;  radiating 
outward  in  concentric  spheres,  if  wholly  unrestrained  ^ 
or  in  sections  of  spheres  or  spheroids,  if  limited  and 
starting  from  impulses  at  various  points  on  any  surface, 
either  spherical,  like  that  of  the  sun,  or  plane. 

Having  taken  an  illustration  from  a  liquid,  illus- 
trations from  solids  will  also  be  in  point. 

If  a  long  rope,  stretched  lengthwise,  with  plenty 
of  slack,  be  held  at  one  end,  and  jerked  rapidly  up  and 
dewn,  it  will  be  thrown  into  wraves,  which  will  run 
along  its  entire  length. 

Here  it  is  evident  that  while  the  impulses  given  at 
one  end  travel  in  waves  to  the  other,  the  rope,  as  a 
whole,  remains  stationary ;  successive  portions  acting 


-> 

TUK  NATURE  OF  ELECTRICITY.  3o 

as  yielding   levers  to    transmit  the  impulse"  along  its 
length. 

If  one  end  of  a  lever  be  depressed  below  a  horizon! 
it  receives  a  forward  as  well  as  downward  movement, 
in  the  arc  of  a  circle,  its  opposite  end  receiving  an  up- 
ward and  backward  movement.  In  this  way  each  suc- 
cessive portion  of  the  rope  oscillates  horizontally  as 
well  as  vertically,  modified  by  the  difference  between  a 
yielding  and  a  rigid  body. 

Let  a  number  of  elastic  balls  be  suspended  in  a 
straight  line  in  contact  with  each  other.  Draw  back 
the  outer  ball  at  one  end  of  the  line  and  let  it  swing 
against  the  adjoining  ball ;  the  impulse  will  be  trans- 
mitted along  the  line,  and  the  outer  ball,  at  the  other 
end,  will  swing  off  to  nearly  the  same  distance  as  that 
through  which  the  first  ball  swung,  all  the  others  re- 
maining stationary. 

Here  the  impulse  is  transferred  from  ball  to  ball  by 
virtue  of  their  elasticity.  When  number  one  impinges 
on  number  two  the  impact  changes  its  shape  slightly  to 
that  of  a  spheroid;  as  it  resumes  its  shape  it  imparts  the 
impulse  to  number  two,  by  which  it  is  imparted  to  num- 
ber three,  and  so  on  through  the  line.  But  action  and 
reaction  being  equal  and  opposite,  there  is  no  perceptible 
movement  till  the  last  ball  is  reached,  which  swings  off, 
since  there  is  no  ball  to  react  against  it.  The  impulse 
travels,  but  the  line  remains  stationary. 

Here,  also,  it  will  be  perceived  that  there  is  a  radial 
force  acting  at  right  angles  to  the  horizontal  force, 
much  the  same  as  would  result  from  a  similar  impact 
if  each  ball  were  hollow,  and  its  surface  composed  of  an 
infinite  number  of  semicircles,  joined  at  the  points  of 
impact. 


86  ELEMENTS  OF  STATIC  ELECTRICITY. 

Now  the  molecules  of  a  metal  rod  may  be  com- 
pared to  an  infinite  number  of  these  lines  of  balls ;  and 
it  may  be  assumed  that  a  heat  impulse,  or  an  electric 
impulse,  given  at  one  end,  moves  along  these  lines 
in  some  way  analogous  to  that  in  which  the  impulse 
moves  along  the  lines  of  balls. 

We  are  not  obliged  to  confine  ourselves  to  any  spe- 
cific method  of  movement;  but  may  suppose  a  wave 
movement,  similar  to  that  which  takes  place  in  the 
slack  rope,  or  on  the  water,  if  it  shall  seem  to  accord 
best  with  known  facts  and  phenomena. 

In  the  polarization  of  light,  it  is  supposed  that  the 
waves  assume  a  certain  phase,  in  conformity  with  the 
special  arrangement  of  the  molecules  of  the  crystal. 
Hence  if  the  crystals  are  cut  from  the  same  block,  and 
placed  in  the  same  position,  the  phase  will  be  the  same 
for  each,  and  the  light  will  pass  through.  But  if  the 
second  is  turned  at  right  angles  to  the  first,  the  phase 
produced  by  passing  through  the  first  will  not  be  in  con- 
formity with  the  arrangement  of  the  molecules  in  the 
second,  and  hence  the  light  cannot  pass  through. 

Suppose  the  arrangement  -of  the  molecules  to  be  in 
layers,  or  strata,  like  the  sheets  composing  a  ream  of 
note-paper,  placed  in  a  vertical  position ;  the  waves 
of  ether  would  assume  a  vertical  phase,  and,  meeting 
the  second  crystal,  placed  in  the  same  position,  would 
pass  through.  But  if  the  second  were  turned,  so  as  to 
bring  its  strata  to  a  horizontal  position,  the  vertical 
waves  would  be  broken,  and  could  not  pass  through. 

Instead  of  the  ether  we  may  suppose  the  molecules 
themselves  thrown  into  waves,  whose  phase  would  con- 
form to  the  structure  of  the  crystal,  and  the  same  result 
would  evidently  follow. 


THE  NATURE  OF  ELECTRICITY.  37 

There  is  no  reference,  in  this  supposed  case,  to  any 
visible  stratification  of  a  crystal,  as  the  experimental 
ray  is  usually  admitted  at  right  angles  to  such  stratifi- 
cation ;  the  reference  is  to  an  invisible  arrangement  of 
the  molecules. 

The  same  course  of  reasoning  is  applicable  to  heat, 
or  to  electricity,  but  the  phase  of  the  heat  wave,  or 
the  electric  wave,  may  be  different  from  that  of  the 
wave  of  light,  so  that  a  substance  opaque  to  light,  as 
copper,  might  allow  the  free  passage  of  heat  or  electric- 
ity ;  or  a  substance  transparent  to  light,  as  glass, 
might  obstruct  their  passage. 

And  the  medium  in  which  the  electric  energy  travels 
may  be  the  ether  which  is  supposed  to  pervade  the 
different  kinds  of  matter ;  or  the  matter  itself,  in  any 
of  its  various  forms,  solid,  liquid,  or  gaseous :  as  it 
has  been  shown  that  undulations'  may  take  place  in 
each  of  them. 

CONDUCTIVITY  FOR  HEAT  AND  ELECTRICITY  COM- 
PARED.— It  is  very  remarkable,  and  must  be  something 
more  than  mere  coincidence,  that  conductivity  for  heat 
and  electricity  is  nearly  the  same  in  the  same  sub- 
stances. A  good  heat  conductor  is  a  good  electric 
conductor;  a  non-conductor  of  heat  is  a  non-conductor 
of  electricity.  So  that  if  we  know  either  the  conduc- 
tivity or  resistance  of  any  substance  for  heat,  we 
have,  practically,  its  conductivity  or  its  resistance  for 
electricity.  This  will  appear  from  the  table  follow- 
ing, showing  the  results  obtained  by  Wiedmann  and 
Franz. 

Hence  if  heat  and  light  are  modes  of  motion,  travers- 
ing various  substances  by  undulations,  we  are  warranted 
in  assuming  the  same  with  reference  to  electricity. 


Substance. 

Heat  Conductivity. 

Silver      .     .     . 

.     .       100 

Copper    .     .     . 
Gold  .... 

.     .        74 
53 

Brass       .     .     . 

.     .         24 

Tin     .... 

15 

Iron    .... 

12 

Lead  .... 

9 

Platinum     .     . 

.     .           8 

German  silver 

.     .          6 

Bismuth 

.     .           2 

38  ELEMENTS  OF  STATIC  ELECTRICITY. 

COMPARATIVE  CONDUCTIVITY  OF  DIFFERENT  SUB- 
STANCES FOR  HEAT  AND  ELECTRICITY,  AS  GIVEN  BY 
WlEDMANN  AND  FRANZ  :— 

Electric  Conductivity. 

100 
73 
59 
22 
23 
13 
11 
10 
6 
2 

Other  observers  place  the  electric  conductivity  of 
some  of  these  substances  much  higher,  making  the  con- 
ductivity of  copper  nearly  equal  to  that  of  silver. 

If  the  electric  wave  has  its  own  peculiar  structure,  it 
is  evident  that  a  substance  whose  structure  is  adapted 
to  it,  or  whose  molecules  easily  adapt  themselves  to  it, 
would  be  a  conductor;  while  a  substance  whose  struct- 
ure is  not  so  adapted,  or  whose  molecules  resist  such 
adaptation,  would  be  a  non-conductor. 

An  attempt  to  insert  a  No.  36  screw  into  a  No.  30 
screw  hole  will  fail,  because  the  threads  of  the  screws 
are  not  adapted  to  each  other.  But  let  the  same  screw 
be  inserted  into  some  yielding  substance,  as  soft  wood, 
and  the  substance  adapts  itself  to  the  structure  of  the 
screw ;  or,  as  we  say,  it  cuts  its  own  thread ;  while  a 
rigid  substance  like  iron  resists  such  adaptation. 

Something  analogous  to  this  may  constitute  the 
difference  between  conductors  and  non-conductors, 
and  may  also  be  the  cause  of  other  electric  phenomena 
of  equal  importance. 


THE  NATURE  OF  ELECTRICITY.  39 

THE  SPAEK  AND  SNAP. — As  already  stated,  every 
substance  offers  a  certain  degree  of  resistance  to  the 
passage  of  electricity,  and  a  result  of  this  resistance 
is  the  generation  of  heat,  often  accompanied  with  light, 
and  this  effect  varies  as  the  resistance. 

Platinum  is  a  metal  of  high  resistance,  while  that  of 
copper  is  very  low;  and  a.  fine  platinum  wire  will  be 
brought  to  a  white  heat  by  an  electric  current  which 
would  scarcely  change  the  temperature  of  a  copper  wire 
of  the  same  size. 

Air  offers  such  high  resistance  that  the  passage  of 
electricity  through  it  always  produces  both  heat  and 
light,  in  the  form  of  a  bright  spark.  This  occurs  not 
only  when  an  electric  charge  passes  through  several 
inches  of  it,  but  through  the  thinnest  film  ;  the  pres- 
ence of  heat,  as  well  as  light,  being  demonstrated  by  the 
lighting  of  gas  by  a  spark  less  than  J  inch  in  length. 

A  sudden  condensation  of  the  air,  forced  forward 
and  laterally  by  the  charge,  has  been  suggested  as 
the  probable  cause.  If  such  condensation  takes  place, 
heat  would  certainly  be  the  result,  as  when  air  is  com- 
pressed by  mechanical  means.  And  perhaps  it  would 
be  accompanied  by  light,  though  this  is  not  probable, 
as  combustion  and  incandescence  are  the  only  known 
means  of  producing  artificial  light  besides  that  now 
under  consideration,  either  of  which  would  imply  the 
presence  of  some  other  substance  besides  air.  But  the 
hypothesis  seems  to  assume  the  passage  of  some  material 
substance  through  the  air  to  produce  the  condensation, 
of  which  there  is  no  evidence. 

But  if,  instead  of  condensation,  we  suppose  undula- 
tions to  take  place,  giving  great  intensity  of  motion, 
as  the  electric  impulse,  darting  forward  with  incon- 


40  ELEMENTS  OF  STATIC  ELECTRICITY, 

ceivable  velocity,  suddenly  forces  the  resisting  air  into 
the  phases  of  the  electric  waves ;  then  the  generation 
of  those  other  modes  of  motion,  known  as  heat  and 
light,  might  easily  be  the  result. 

A  sharp  sound,  varying  from  an  insignificant  snap  to 
a  deafening  report,  always  accompanies  the  spark.  On 
the  condensation  theory  this  is  accounted  for  by  the 
sudden  displacement  and  reflux  of  the  air.  But  since 
sound,  like  heat  and  light,  is  another  mode  of  motion, 
occurring  chiefly  in  the  air,  it  is  evident  that  the  wave 
theory  will  best  account  for  it ;  the  electric  impulse 
giving  rise  to  these  different  modes  of  motion. 

THE  DUAL  THEORY. — We  have  already  seen,  in  ex- 
periments with  the  pith  ball  electroscope,  that  the  balls 
may  be  attracted  and  repelled  by  electrified  glass,  seal- 
ing- wax,  and  various  other  substances. 

Let  an  electrified  glass  rod  approach  one  of  the  balls  ; 
the  ball  is  attracted,  and,  after  contact,  repelled.  Now 
let  an  electrified  stick  of  sealing-wax  be  brought  near, 
and  the  electrified  ball,  which  was  repelled  by  the  glass, 
is  attracted  by  the  wax.  Or  let  the  ball  be  first  elec- 
trified and  repelled  by  the  wax,  and  it  is  attracted  by 
the  glass. 

Further  experiments  show  that  the  same  results  can 
be  produced  with  other  substances;  glass  representing 
a  certain  class  of  substances,  which  show  similar  electri- 
fication, and  sealing-wax  and  resin  another  class,  which 
shows  opposite  electrification  to  that  of  glass. 

Hence  it  has  been  assumed  that  there  are  two  kinds  of 
electricity.  One  kind  generated  on  the  class  of  sub- 
stances represented  by  glass,  and  the  other  on  the  class 
represented  by  resin.  The  former  was  once  designated 
as  vitreous,  and  the  latter  as  resinous;  but  the  term 


THE  NATURE  OF  ELECTRICITY,  41 

'    . 

positive  is  now  used  instead  of  vitreous,  and  negative 
instead  of  resinous.  Used  in  this  way,  these  terms 
have  no  reference  to  a  difference  either  in  quantity  or 
intensity  ;  they  express  only  a  supposed  difference  in 
kind,  not  in  degree. 

-This  doctrine  of  the  dual  nature  of  electricity  was 
first  proposed  by  Dufaye,  and  has  since  been  strongly 
maintained  by  many  eminent  scientists.  Deschanel, 
speaking  of  the  phenomena  under  consideration,  says  : 
"  These  phenomena  clearly  show  that  the  electricity  de- 
veloped on  the  resin  is  not  of  the  same  kind  as  the  elec- 
tricity developed  on  the  glass." 

Now  the  only  thing  "clearly  shown  "  is  the  difference 
in  the  substances,  not  in  the  electricity.  For  we  have 
precisely  the  same  electric  phenomena  of  attraction 
and  repulsion  with  the  glass  as  with  the  sealing-wax ; 
but  a  third  substance,  the  electrified  pith  ball,  is  at- 
tracted by  one  and  repelled  by  the  other ;  a  result 
which  it  would  seem  more  reasonable  to  attribute  to 
the  difference  known  to  exist  between  the  substances, 
than  to  a  difference  supposed  to  exist  in  the  electricity. 
For  it  has  already  been  shown  that  different  causes,  as 
conductivity  or  resistance,  influence  the  intensity  of 
electrification  on  different  substances.  Other  causes 
also,  as  a  difference  of  temperature  or  mass,  of  hard- 
ness or  softness,  density  or  porosity,  doubtless  contribute 
to  the  same  result. 

But  considering  the  quality  of  resistance  alone,  the 
potential  of  any  non-conductor,  as  glass,  is  liable  to 
vary  greatly  on  different  parts  of  its  surface,  when 
electrified  by  friction  ;  and  to  differ  from  the  potential 
of  sealing-wax,  similarly  produced  on  different  parts 
of  its  surface. 


42  ELEMENTS  OF  STATIC  ELECTRICITY. 

The  friction  of  the  same  rubber  is  also  greater  on 
a  substance  like  sealing-wax,  whose  surface  is  soon  soft- 
ened by  the  heat  generated,  than  on  a  smooth,  hard  sub- 
stance like  glass,  which  is  not  affected  in  this  way. 

And,  as  already  shown,  difference  of  potential  produces 
attraction,  while  equality  of  potential  produces  repulsion, 
between  bodies.  Hence  the  attraction  by  the  sealing- 
wax,  of  the  pith  ball  electrified  by  the  glass,  is  a  neces- 
sary result  of  difference  of  potential ;  while  its  repul- 
sion by  the  glass  follows  from  equality  of  potential. 
And  the  same  will  be  true  of  the  ball  electrified  and  re- 
pelled by  the  wax  and  attracted  by  the  glass. 

But  if  a  difference  exists  in  the  kind  of  electricity 
produced  by  the  different  classes  of  substances,  we 
should  expect  that  difference  always  to  manifest  itself 
whenever  one  of  either  class  is  employed  as  a  generator. 

But  this  is  only  true  in  a  general  way,  to  which  the 
exceptions  are  very  numerous  ;  for  it  often  happens  that 
glass  and  sealing-wax,  or  other  substances  belonging  to 
the  different  classes,  when  rubbed  with  the  same  rubber, 
exhibit  the  same  electric  qualities.  The  same  result 
also  will  often  follow  \vhere  different  kinds  of  rubbers 
are  employed,  as  silk  on  one  substance  and  woolen  on 
the  other. 

Such  results  are  inconsistent  with  the  theory  of  two 
electricities  ;  but  are  easily  accounted  for  by  a  differ- 
ence, or  an  equality  of  potential,  which  we  know  is 
liable  to  exist. 

Hence,  preference  must  be  given  to  the  doctrine  of  one 
electricity,  originally  proposed  by  Franklin  ;  simple  and 
plain,  like  truth  itself,  and  in  strict  accord  with  all  elec- 
tric phenomena ;  whether  pertaining  to  static  electric- 
ity, or  to  electricity  under  other  forms. 


CHAPTER  IV. 
INDUCTION. 

IT  is  noticeable  that  in  all  our  experiments  thus  far 
the  electrified  body  acts  on  the  other  bodies  before  there 
is  any  actual  contact.  The  knife-handle  attracts  the 
spoon  ;  the  sealing-wax,  ebonite,  or  glass  attracts  the 
balanced  rod  or  the  pith  ball  while  separated  from  them. 
And  when  either  of  these  electrified  bodies  approaches 
the  gold-leaf  electroscope,  there  is  first  a  divergence  of 
the  leaves  before  contact  occurs. 

It  will  also  be  noticed  that  this  effect  increases  or 
diminishes  as  the  distance  is  increased  or  diminished. 
And,  further,  that  while  the  interposition  of  different 
substances,  as  glass,  paraffin,  ebonite,  air,  wood,  metal, 
produce  great  variations  in  the  effect,  none  of  them 
wholly  prevent  it. 

There  is  evidently,  then,  an  invisible  influence  ex- 
tending to  a  certain  distance  from  the  electrified  body 
in  every  direction,  and  affecting  everything  within  its 
sphere,  and  this  effect  is  called  induction. 

When  an  electrified  body  is  brought  near  the  disc  of 
the  electroscope  without  touching  it,  the  leaves  diverge, 
and  on  its  removal  converge  again,  showing  no  perma- 
nent effect.  But  if  it  is  allowed  to  touch  the  disc,  the 
leaves  are  electrified,  and  remain  divergent  after  its 
removal. 

But  if,  instead  of  touching  the  disc,  it  be  held  near 


44 


ELEMENTS   OF  STATIC  ELECTRICITY. 


enough  to  produce  divergence,  as  at  J.,  Fig.  5,  and,  while 
in  that  position,  the  disc  be  touched  with  the  finger,  as 
at  B,  the  leaves  will  converge,  and  remain  so  as  long 
as  the  electrified  body  is  held  near ;  but  on  its  removal 
as  at  (7,  they  will  diverge,  and  remain  divergent,  the 
same  as  after  contact  of  the  electrified  body  with  the 
disc. 


Fig.  5 — Induction  Illustrated. 

Here,  then,  is  electrification  by  induction,  without 
any  transfer  of  electricity  by  contact.  How  can  this  be 
accounted  for  ? 

When  the  electrified  body  is  brought  near,  whether 
its  charge  be  positive  or  negative,  the  effect  of  induction 
is  to  produce  a  temporary  change. of  the  potential  of 
the  electroscope,  and  the  leaves  diverge. 

If  the  charge  of  the  electrified  body  be  positive,  elec- 
tricity is  repelled  from  the  disc  to  the  leaves,  and  they 
diverge,  being  positively  electrified  to  the  same  poten- 
tial, and  hence  mutually  repellent,  and  also  attracted 
by  the  lower  potential  of  surrounding  bodies. 

But   if  the   electrified    body  be    negatively  charged, 


INDUCTION.  45 

electricity  is  attracted  from  the  leaves  to  the  disc,  and 
they  diverge,  being  negatively  electrified,  and  mutually 
repellent,  as  before,  and  attracted  by  the  higher  poten- 
tial of  surrounding  bodies. 

Now,  when  the  disc  is  touched  with  the  finger,  and 
thus  connected  with  the  earth,  if  the  charge  is  positive, 
the  potential  of  the  electroscope  is  changed  by  the 
escape  of  electricity  to  the  earth  under  the  influence  of 
the  electrified  body,  and  the  leaves  converge.  But  if 
the  charge  is  negative,  the  potential  of  the  electroscope 
is  changed  by  the  attraction  of  electricity  from  the  earth, 
and  the  leaves  converge  as  before,  equilibrium  being  re- 
stored between  the  disc  and  leaves  in  each  case. 

The  leaves  remain  convei  gent  so  long  as  the  electrified 
body  is  held  near  ;  the  electroscope  being  still  under  the 
influence  of  the  force  by  which  the  change  of  potential 
was  produced  ;  which  is  evidently  just  equal  to  the  re- 
pelled, energy  in  the  first  instance,  and  to  the  attracted 
energy  in  the  second.  But  when  the  electrified  body  is 
removed,  this  equilibrium  is  disturbed,  and  the  leaves 
diverge  under  the  influence  of  mutual  repulsion  and 
outward  attraction,  as  already  explained. 

This  experiment  proves  that  a  body  connected  with 
the  earth,  and  under  the  influence  of  induction,  may 
differ  in  potential  from  the  earth,  and  is  not  necessarily 
at  zero  potential  from  its  earth  connection.  For  it  is 
evident  that  such  difference  of  potential  existed  during 
the  connection  of  the  electroscope  with  the  earth,  else 
it  could  not  have  become  manifest  when  the  connection 
was  severed  and  the  inductive  influence  removed..  For 
when  the  electrified  body  is  removed  before  such  con- 
nection, the  leaves  converge,  but  when  removed  after  it 
has  been  made  and  severed,  they  remain  divergent; 


46  ELEMENTS  OF  STATIC  ELECTRICITY. 

showing  that  the  difference  of  potential  was  created  and 
existed  during  the  earth  connection. 

This  point  has  an  important  bearing  on  phenomena  to 
be  considered  hereafter,  in  regard  to  which  prominent 
writers  have  been  betrayed  into  serious  mistakes  from 
having  overlooked  it. 

INFLUENCE  OF  DISTANCE. — It  is  important  to  notice, 
that  when  the  electroscope  has  been  charged  by  induc- 
tion in  this  manner,  and  the  electrified  body  is  again 
brought  near,  the  leaves  continue  to  converge  as  the 
body  approaches,  and  come  together  when  it  is  in  the 
same  position  as  when  the  disc  was  touched  with  the 
finger.  A  nearer  approach  produces  divergence,  which 
increases  as  the  body  is  brought  still  nearer. 

Let  it  now  be  gradually  withdrawn,  and  the  leaves 
gradually  converge  and  come  together,  when  the  body 
reaches  the  same  point,  as  before.  Further  withdrawal 
produces  divergence,  which  continues  to  increase,  and 
reaches  its  limit  when  the  body  is  wholly  removed. 

If  the  electroscope  be  placed  at  various  points,  equally 
distant  from  the  electrified  body,  the  effect  of  induction 
will  be  the  same,  so  long  as  the  same  distance  from  the 
earth  and  surrounding  objects  is  maintained.  Hence 
it  is  evident  that  electric  energy,  like  other  forms  of 
radiant  energy,  as  light  and  heat,  radiates  equally 
in  all  directions  when  not  interfered  with  by  other 
influences. 

Suppose  the  electrified  body  to  be  a  small  globe, 
entirely  removed  from  the  earth,  and  surrounded  with 
a  perfectly  homogeneous  medium,  it  would  be  the 
center  of  a  sphere  of  inductive  influence.  And  suppose 
the  lines  of  force  radiating  from  it  to  be  cut  by  the 
surfaces  of  two  imaginary  concentric  spheres  of  differ- 


INDUCTION.  47 

ent  sizes,  one  placed  outside  of  the  other,  and  having 
the  electrified  globe  for  their  common  center. 

Since  the  surfaces  of  spheres  are  to  each  other 
as  the  squares  of  their  radii,  and  since  the  radii  meas- 
ure the  distances  from  the  center,  the  surfaces  are  to 
each  other  as  the  squares  of  their  distances  from  the 
center. 

But  as  each  surface  embraces  all  the  lines  of  force, 
the  intensity  of  force  on  equal  surface  areas  of  the  two 
spheres  would  be  in  the  inverse  ratio  of  their  entire  sur- 
faces ;  and  hence  would  vary  inversely  as  the  squares 
of  their  distances  from  the  center. 

Hence,  electric  induction  varies  inversely  as  the  square 
of  the  distance. 

Practically  the  conditions  of  the  supposed  case  are 
never  exactly  fulfilled ;  but  that  does  not  affect  the 
correctness  of  the  principle,  which  is  the  same  in  elec- 
tricity as  in  light  and  radiant  heat. 

CYLINDER  ELECTRIFIED  BY  INDUCTION. — The  effect 
of  induction  may  be  further  illustrated  by  an  insulated 
cylinder  of  conducting  material,  placed  between  two 
spheres  of  similar  material,  one  insulated,  and  the  other 
connected  with  the  earth  by  a  chain,  as  shown  in  Fig. 
6 ;  the  cylinder  having  mounted  on  it  three  pith-ball 
electroscopes,  connected  with  it  by  conductors. 

If  the  insulated  sphere,  A,  be  positively  electrified, 
electricity  will  be  repelled  by  induction  from  the  end  of 
the  cylinder  next  A  to  the  end  next  B.  And  since  B 
is  connected  with  the  earth,  the  electricity  accumulated 
on  the  end  of  (7,  next  to  it,  will  repel  to  the  earth  from 
B  an  amount  equal  to  the  positive  charge  on  A. 

Hence  the  pith  ball  next  A,  being  negative  and  A 
positive,  is  attracted  by  A,  while  the  one  next  B,  being 


48  ELEMENTS  OF  STATIC  ELECTRICITY. 

positive  and  B  negative,  is  attracted  by  B ;   but  the 
central  ball,  being  neutral,  remains  unmoved. 

If  the  sphere,  A,  be  negatively  electrified,  these  condi- 
tions will  all  be  reversed.  Electricity  will  be  attracted 
to  the  end  of  C  next  A,  and  a  positive  charge,  equal 
to  the  negative  on  J.,  attracted  from  the  earth  to  B. 
Hence  the  balls  will  assume  the  same  positions  as 
before. 


Fig.  G — Cylinder  Electrified  by  Induction. 

Similar  inductive  effects  can  be  produced  on  the  cyl- 
inder by  the  sphere  A  alone,  but  less  marked  than  when 
two  spheres  are  used;  and,  for  such  an  experiment,  tin- 
foil electroscopes  are  better  than  those  made  with  pith 
balls,  being  more  sensitive. 

THEORY  OF  INDUCTION. — It  Is  noc  known  how  in- 
ductive force  is  transmitted.  The  hypothesis  has  been 
advanced  that  it  is  by  a  certain  strain  of  the  medium ; 
as  when  a  weight  is  lifted  by  a  rope  or  pushed  by  a  pole, 
the  energy  is  transmitted  in  one  case  by  the  tension  of 
successive  portions  of  the  rope,  and  in  the  other  by  a 
compression  of  successive  portions  of  the  pole.  In 
either  case  the  energy  or  stress  produces  a  strain,  which 


INDUCTION.  49 

runs  through  the  substance  of  the  medium  till  it  reaches 
the  object,  and  the  continued  stress  produces  continued 
strain.  Something  analogous  to  this,  it  is  assumed, 
takes  place  in  the  transmission  of  electric  energy  by 
induction. 

This  hypothesis  has  the  sanction  of  eminent  author- 
ity, and  may  assist  us  in  arriving  at  a  solution  of  the 
problem.  Gordon  says:  "If  electric  induction  were  a 
4  direct  action  at  a  distance,'  we  should  expect  that  it 
would  be  transmitted  equally  through  all  insulators. 
One  of  the  strongest  arguments  for  supposing  it  to  be 
a  strain  of  the  particles  of  the  insulator  is  found  in  the 
fact  that  different  insulators  transmit  it  with  very  differ- 
ent strengths." 

"Induction,  so  far  from  being  a  'direct  action  at  a 
distance,'  is  most  certainly  transmitted  by  the  particles 
of  the  dielectrics,  and  is  affected  by  almost  every  molec- 
ular change  which  may  occur  in  them." 

And  he  defines  strain,  as  here  used,  to  mean  "  an 
alteration  of  size  or  shape,"  including  "all  alterations 
of  volume,"  "  all  twistings  and  bendings,  and  all  vibra- 
tory motions  other  than  those  of  a  rigid  body  as  a 
whole." 

The  wave  theory  agrees  with  the  views  here  expressed ; 
for  we  have  only  to  conceive  that  this  "  strain  "  consists 
in  a  "  vibratory  motion,"  that  is,  in  undulations  of  the 
medium. 

It  is  also  in  accordance  with  the  analogy  of  similar 
transmission  of  other  forms  of  radiant  energy.  And,  if 
all  energy  has  a  common  origin,  it  is  reasonable  to  sup- 
pose that  the  transmission  of  its  different  forms  would 
present  striking  analogies. 

INFLUENCE  OF  DIELECTRIC. — In  order  to  observe 


50  ELEMENTS  OF  STATIC  ELECTRICITY. 

induction  there  must  be  two  or  more  bodies  at  differed 
potentials  placed  in  each  other's  vicinity,  and  these  must 
be  separated  by  an  insulator;  for,  if  separated  by  a  con- 
ductor, equilibrium  would  at  once  be  restored,  and  in- 
duction could  not  take  place. 

Insulators  through  which  induction  takes  place  are 
called  dielectrics,  from  the  Greek  &«,  through.  Air  was 
the  dielectric  between  the  electroscope  and  electrified 
body,  and  between  the  spheres  and  cylinder,  in  the  ex- 
periments already  given. 

Now,  since  conductors  permit  electricity  to  pass 
through  them  easily,  while  insulators  resist  its  passage, 
there  must  be  some  peculiarity  in  the  nature  or  arrange- 
ment of  the  molecules  which  makes  two  bodies  of  the 
same  class  similar  in  this  respect,  while  two  of  opposite 
classes  are  dissimilar. 

Hence  we  can  easily  conceive  that  when  two  insulated 
conductors,  at  different  potentials,  are  brought  into  con- 
tact, the  undulations  of  their  molecules  would  assume 
the  same  phase,  and  equilibrium  take  place;  but  that 
when  those  undulations  are  transmitted  through  a  die- 
lectric, they  undergo  such  a  change  that,  the  phases  of 
the  undulations  not  being  the  same,  there  is  a  repulsion 
instead  of  an  intermingling,  which  results  in  creating 
opposite  potentials  in  adjacent  parts,  on  either  side  of 
the  dielectric,  the  negative  of  one  being  equal  to  the 
positive  of  the  other. 

And  since  in  the  transmission,  part  of  the  energy  is 
consumed  in  overcoming  the  resistance,  difference  of 
potential,  on  opposite  sides,  must  result  from  this  cause 
also. 

If  either  conductor  be  removed,  still  remaining  insu- 
lated, the  equilibrium  of  each  will  be  restored,  and  its 


INDUCTION.  51 

potential  be  found  the  same  as  before  it  was  brought 
within  the  sphere  of  inductive  influence,  showing  that 
no  permanent  effect  has  resulted. 

Hence  it  will  be  seen  that  the  effect  of  induction  is 
opposite  to  that  of  contact;  the  latter  producing  perma- 
nent equilibrium  between  conductors,  while  the  former 
produces  temporary  disturbance  of  equilibrium. 

SPECIFIC  INDUCTIVE  CAPACITY. — It  has  already  been 
stated  that  electric  induction  takes  place  through  all 
substances,  but  in  different  degrees ;  and,  since  it 
is  found  that  each  has  an  inductive  power  peculiar 
to  itself,  this  property  is  called  its  specific  inductive 
capacity. 

The  importance  of  this  subject  will  be  understood 
when  it  is  considered  that  it  affects  enterprises  in- 
volving large  capital,  public  convenience,  and  public 
safety ;  as  in  the  transmission  of  electric  energy  by 
insulated  conductors,  as  telegraph  and  telephone  wires, 
ocean  cables,  and  electric  light  wires  ;  including  the 
important  question  of  underground  transmission  in 
cities. 

Hence,  for  the  last  forty  years,  it  has  engaged  the  atten- 
tion of  such  men  as  Faraday,  Boltzmann,  and  many 
others,  including  the  earlier  researches  of  Cavendish, 
who  seems  to  have  been  the  first  to  investigate  it,  but 
whose  experiments  on  this  subject  have  only  recently 
been  published. 

The  general  method  of  investigation  is  as  follows: — 
The  inductive  capacity  of  dry  air  at  the  barometric 
pressure  of  760  millimeters  (29.92  inches)  and  at  the 
temperature  of  0°  C.  (32°  Fahrenheit)  is  made  the 
standard  unit  by  which  the  inductive  capacities  of  all 
other  substances  are  estimated. 


52  ELEMENTS  OF  STATIC  ELECTRICITY. 

To  illustrate  : — Suppose  we  have  two  insulated  metal 
plates,  A  and  B  (Fig.  7j,  separated  by  an  air  space  C ' ; 
let  A  be  electrified  and  J9.  connected  with  the  disc  of 
an  electroscope.  First  note  the  amount  of  divergence 
of  the  leaves ;  then  let  a  plate  of  glass,  cake  of  paraffin, 
or  any  other  insulator  which  will  exactly  fill  the  space 


Fig.  7 — Specific  Inductive  Capacity  Illustrated. 

(7,  be  introduced  between  the  plates,  and  note  the  diver- 
gence of  the  leaves  now,  as  compared  with  the  former 
divergence. 

As  this  insulator  has  displaced  the  air,  it  is  evident 
that  its  inductive  capacity,  as  compared  with  air,  is 
shown  by  the  difference  in  the  divergence  of  the  leaves. 


INDUCTION.  53 

If  that  divergence  has  increased,  then  the  power  of  this 
insulator  to  transmit  electric  influence — that  is,  its  spe- 
cific inductive  capacity — is  greater  than  that  of  air; 
otherwise,  it  is  equal  to,  or  less  than  that  of  air. 

From  this  we  see  that  specific  inductive  capacity  varies 
inversely  as  insulation.  Hence  this  property  is  almost 
infinite  in  the  best  conductors ;  while  in  the  best  insu- 
lators it  is  the  reverse. 

By  methods  similar  to  the  above,  with  the  aid  of 
improved  instruments,  to  be  described  hereafter,  the 
specific  inductive  capacities  of  a  number  of  substances, 
including  the  principal  insulators,  have  been  carefully 
estimated  by  Boltzmann,  Gordon,  and  others:  and  from 
the  results  obtained  by  them  the  table  on  the  next  page 
has  been  prepared,  in  which  the  general  averages  are 
given. 

The  results  obtained  by  different  observers  differ  so 
widely  that  they  can  only  be  regarded  as  approximate, 
and  will  undoubtedly  require  future  correction,  when 
improved  methods  shall  give  greater  accuracy. 

The  table  shows  the  electric  resistance  of  glass  to  be 
much  less  than  that  of  ebonite ;  the  inverse  ratio  being 
5.87  to  2  89 :  and  this  is  doubtless  true  of  glass,  in  the 
average.  But,  if  the  best  insulating  glass  were  com- 
pared with  the  best  insulating  ebonite,  the  ratio  might 
require  to  be  reversed.  Ebonite,  when  subjected  to  a 
powerful  electric  strain,  seems  to  yield  gradually,  and 
allow  the  electricity  to  creep  through  it ;  and,  by  con- 
tinued strain,  its  electric  resistance  soon  becomes 
permanently  impaired :  while  the  best  insulating  glass 
rigidly  resists,  and  suffers  fracture  before  yielding. 

But,  according  to  Gordon,  the  electric  resistance  of 
glass  also  becomes  somewhat  impaired  by  long  use  ;  or, 


54  ELEMENTS  OF  STATIC  ELECTRICITY. 

which  is  the  same  thing,  its  specific  inductive  capacity 
is  increased.  All  of  which  goes  to  prove  that  electric 
transmission  depends  on  molecular  structure. 


SPECIFIC  INDUCTIVE  CAPACITIES  OF  VARIOUS 
SUBSTANCES. 

Standard. 

Air  at  0°  C.  temperature  and  760  mm.  pressure,  1.0 

Solids. 

Paraffin, 2.09 

Caoutchouc, 2.23 

Gutta-percha, 2.46 

Shellac, 2.85 

Ebonite, 289 

Sulphur,    .     .• 2.95 

Resin,    . .     .  3.6 

Glass,  average  of  various  kinds, 5.87 

Liquids. 

Bisulphide  of  carbon, 1.81 

Petroleum, 2.05 

Oil  of  turpentine, 2.19 

Gases. 

Hydrogen,  H,      at  0°  C.  and  760  mm.,  .99941 

Carbonic  oxide,         CO,          "                  "  1.00001 

Marsh  gas,                  CH4,         «  1.00035 

Carbonic  dioxide,      CO2,         "  1  00036 

Nitrous  oxide,           NO,          "  1.00039 

Olefiant  gas,              C2H4,        "  1.00072 


CHAPTER  V. 
ELECTKIC  DISTRIBUTION  AND  CONDENSATION. 

EQUIPOTENTIAL. — A  charge  of  electricity  given  to 
any  part  of  a  good  conducting  surface  is  instantly  dis- 
tributed equally  over  every  part,  and  such  a  surface  is 
called  equipotential.  For  the  momentary  increase  of 
electric  energy  at  any  point  creates  electric  movement 
from  higher  to  lower  potential,  which  instantly  results 
in  the  establishment  of  equilibrium  at  every  point. 

Separate  points  on  such  a  surface  are  called  equipo- 
tential points,  and  a  line  of  such  points  an  equipotential 
line. 

LINES  OF  FORCE. — The  direction  along  which  elec- 
tricity tends  to  move,  from  a  point  of  higher  to  one  of 
lower  potential,  is  called  a  line  of  force.  Such  lines 
are  perpendicular  to  the  equipotential  surfaces  at  the 
points  ;  for,  as  the  tendency  is  to  move  from  one  point 
to  the  other,  it  would  be  from  one  such  surface  to  the 
other ;  and  if  the  line  differed  from  a  perpendicular,  it 
would  imply,  by  the  resolution  of  forces,  that  there 
could  be  two  lines  of  force  at  right  angles  to  each 
other,  one  of  which  would  lie  in  an  equipotential  sur-. 
face  ;  implying  two  points  at  different  potentials  in  such 
surface,  which  would  be  an  impossibility. 

SURFACE  CONDENSATION. — Since  the  surface  of  a 
solid  sphere  of  any  good  conducting  material  is  evi- 
dently equipotential,  we  may  regard  its  interior  as 


56  ELEMENTS  OF  STATIC  ELECTRICITY. 

composed  of  an  infinite  number  of  such  surfaces,  or 
spherical  shells,  having  a  common  center ;  and  their 
radii  as  equipotential  lines  cut  by  such  surfaces.  From 
which  it  is  evident  that  no  difference  of  potential  could 
exist  in  the  interior  of  such  a  sphere. 

If  it  were  insulated,  a  positive  charge  communicated 
to  it  would  evidently  be  distributed  equally  through 
every  part,  if  there  were  no  influence  tending  to  pro- 
dace  a  different  effect.  But,  since  the  sphere  would 
be  at  a  higher  potential  than  its  surroundings,  induc- 
tion would  create  lines  of  force  in  the  direction  of  the 
radii,  which  must  result  in  the  condensation  of  the  en- 
tire charge  on  the  surface. 

Also,  since  every  portion  of  the  sphere  is  at  the 
same  potential,  and  since  electrified  bodies  at  the  same 
potential  repel  each  other,  it  is  evident  that  the  mole- 
cules would  be  self-repellent.  But  since  they  are  rigid, 
the  electricity  of  each  molecule  would  repel  that  of 
every  other,  and  move  in  the  direction  of  least  resist- 
ance. Let  a  row  of  molecules  composing  a  diameter 
be  selected,  the  direction  of  least  resistance  would  be 
from  the  center  each  way.  For,  if  surface  condensa- 
tion takes  place  (and  experiment  shows  that  it  does), 
as  the  electricity  of  the  molecules  near  each  end  of  the 
diameter  became  condensed  at  the  extreme  points,  its 
reaction  being  thus  neutralized,  more  would  be  repelled 
from  the  center,  and.  this  would  continue  till  all  the 
electricity  of  the  diameter  was  condensed  at  the  ends. 

But  since  the  ends  are  points  on  the  surface,  and  the 
surface  is  made  up  of  an  infinite  number  of  such  points, 
it  is  evident  that  the  entire  charge  would  be  condensed 
on  the  surface. 

Hence  surface  condensation  takes   place  under   the 


ELECTRIC  DISTRIBUTION  AND  CONDENSATION.        57 

influence  of  attraction  from  without  and  repulsion  from 
within,  in  the  direction  of  the  radii. 

If  the  charge  be  negative,  the  potential  of  surround- 
ing bodies  being  higher  than  that  of  the  sphere,  elec- 
tricity is,  in  like  manner,  repelled  from  the  surface 
toward  the  center  ;  and  the  negative  charge  takes  place 
on  the  surface,  as  the  positive  charge  did  in  the  first 
instance.  Hence  the  condensation  is  now  in  the  inte- 
rior, leaving  the  surface  negative. 

Hence  surface  charge,  if  positive,  takes  place  under 
the  influence  of  attraction  from  without  and  repulsion 
from  within ;  but,  if  negative,  under  the  influence  of 
repulsion  from  without. 

In  either  case  the  air  is  the  dielectric  between  the 
electrified  sphere  and  surrounding  bodies :  and  when 
the  charge  on  the  sphere  is  positive,  a  negative  charge 
of  corresponding  amount  is  induced  on  adjacent  parts 
of  surrounding  bodies ;  electricity  being  repelled  from 
them  by  the  higher  potential  of  the  sphere.  But  when 
the  charge  on  the  sphere  is  negative,  the  charge  on 
adjacent  parts  of  surrounding  bodies  is  positive ;  elec- 
tricity being  attracted  to  them  by  the  lower  potential 
of  the  sphere. 

Now  since  surrounding  bodies,  as  a  whole,  are  at 
zero,  and  this  positive  charge,  in  their  adjacent  parts, 
results  from  the  negative  attraction  of  the  sphere,  it 
is  evident  that  the  interior  potential  of  the  sphere,  as  a 
whole,  cannot  rise  above  zero;  the  negative  potential 
of  its  surface  being  exactly  equal  to  the  positive  of 
adjacent  parts  of  surrounding  bodies,  just  as  their 
negative  potential  was  equal  to  the  sphere's  positive 
surface  potential  in  the  first  instance.  Now,  since  a 
solid  of  any  conceivable  shape  could  be  cut  from  such  a 


58  ELEMENTS  OF  STATIC  ELECTRICITY. 

sphere  without  altering  the  electrical  conditions  named, 
it  is  evident  that,  A  charge  of  electricity  communicated 
to  any  solid  conductor  will  be  condensed  on  its  surface. 

SURFACE  TRANSMISSION. — It  is  also  evident,  that 
although  a  static  charge  will  be  thus  condensed  on  the 
surface,  electric  transmission  is  not  confined  to  the  sur- 
face ;  since  surface  condensation  is  due  to  induction 
and  repulsion,  which  implies  the  possibility  of  trans- 
mission through  the  substance  to  reach  the  surface. 

Hence,  although  induction  operates  during  transmis- 
sion, it  cannot  prevent  transmission  through  the  sub- 
stance :  so  that  it  must  not  be  inferred  that  the  con- 
ducting power  is  in  proportion  to  the  surface,  but  to  the 
mass  of  the  conductor. 

Hence  a  charge  of  electricity  which  could  be  easily 
transmitted  by  a  solid  rod  might  be  sufficient  to  melt 
a  thin  tube  of  the  same  diameter. 

HOLLOW  CONDUCTORS. — The  same  reasoning  which 
applies  to  an  electric  charge  on  a  solid  sphere  will  also 
apply  to  one  on  a  hollow  sphere.  For  if  any  number  of 
the  spherical  shells  composing  the  interior  be  removed, 
it  does  not  alter  the  equipotential  of  the  remaining 
ones,  nor  of  their  radii ;  neither  can  it  change  the  induc- 
tion of  the  outside  surroundings. 

And  as  the  form  may  be  altered  without  changing 
these  electric  conditions,  the  same  reasoning  will  apply 
to  any  hollow  conductor. 

Hence,  A  static  electric  charge,  communicated  to  a  hol- 
low conductor,  will  be  condensed  on  its  external  surface. 

PROOF  PLANE — But  all  our  conclusions  should  be 
the  result  of  experiment ;  to  aid  us  in  which  we  now 
require  the  little  instrument  called  the  proof  plane, 
represented  in  Fig.  8 ;  which  consists  of  a  small  brass 


ELECTRIC  DISTRIBUTION  AND  CONDENSATION.        59 


disc,  two  inches  in  diameter,  to  which  is  attached  a 
light  ebonite  handle,  12  inches  long.  A  light,  flat 
spring,  which  lies  close  to  the  disc,  its  lower  end  free, 
and  its  upper  end  attached  to  the  handle,  will  be  found 
convenient  for  attaching  tin-foil  in  some  experiments. 


Fig.  8— Proof  Plane. 

The  proof  plane  is  used  for  examining  the  electric 
condition  of  bodies,  and  for  transferring  a  small  charge 
of  definite  amount.  Care  should  be  used  to  prevent 
the  handle  from  becoming  charged,  which  may  happen 
from  friction  against  the  clothing  or  otherwise. 

EXPERIMENTS  WITH  HOLLOW  CONDUCTORS. — Let  a 
charge  of  electricity  be  given  to  the  insulated  sphere  A, 
Fig.  9,  which  has  an 
opening  in  the  top.  In- 
troduce the  proof  plane 
through  this  opening, 
taking  care  to  prevent 
contact  with  the  edges  ; 
and  touch  the  inside  sur- 
face and  then  the  disc 
of  the  electroscope,  with 
it.  As  the  leaves  show 
no  divergence,  it  proves 
that  the  inside  is  not 
electrified. 

Now  touch  the  outside,  and  then  the  disc,  and  the 
leaves  diverge ;  proving  that  the  charge  is  on  the  out- 
side surface. 

Apply  the  same  tests  to  the  insulated  cylinder  B,  and 


GO  ELEMENTS  OF  STATIC  ELECTRICITY. 

the  same  results  will  follow.  And  this  cylinder  may 
be  composed  either  of  sheet  metal  or  wire  gauze  without 
affecting  the  results. 

Cylinders  of  the  latter  kind  are  often  used  to  protect 
electroscopes  from  the  induction  of  electrified  bodies 
in  their  vicinity. 

Repeat  these  experiments,  communicating  the  charge 
to  the  inside  surfaces  of  the  globe  and  cylinder,  and  the 
results  will  be  the  same ;  showing  that  no  charge  can 
remain  on  the  inside. 


Fig.  10— Faraday's  Bag. 


BAG  EXPERIMENT. — To  test  this  more  thoroughly, 
Faraday  constructed  a  cone-shaped  linen  bag,  shown  in 
Fig.  10 ;  attached  to  its  mouth  a  ring  insulated  on  a 
stand,  and  to  its  apex  two  silk  cords,  by  which  either 
surface  could  be  turned  outward. 

An  electric  charge  was  communicated  to  it,  and,  on 
testing  with  the  proof  plane  and  electroscope,  was  found 
to  be  entirely  on  the  outer  surface.  The  surfaces  were 
now  reversed,  and  the  charge  was  found  to  have  been 
reversed  also,  going  to  the  outside,  as  before. 

PAIL  EXPERIMENT. — The  following  experiment  by 
Faraday  shows  the  effect  of  induction  on  a  hollow  con- 
ductor : 

Let  a  tin  pail  J.,  Fig.  11,  or  any  similar  hollow  con- 


ELECTRIC  DISTRIBUTION  AND  CONDENSATION. 


61 


ductor,  be  insulated  and  connected  by  a  wire  with  an 
electroscope  U,  and  let  an  electrified  metal  ball  B  be 
lowered  into  it  by  a  silk  cord.  The  leaves  will  diverge 
as  the  ball  enters,  and  the  divergence  increase  till  the 
ball  has  passed  some  distance  below '  the  edges  :  after 
which  the  divergence  is  not  increased  by  its  further 
descent. 


Fig.  11— Pail  Experiment. 

If  it  be  lifted  out  without  having  touched  the  pail, 
the  leaves  will  converge,  and  the  ball  show  no  loss  of 
charge  :  but,  if  allowed  to  touch  while  below  the  edge, 


62  ELEMENTS  OF  STATIC  ELECTRICITY. 

the  leaves  will  remain  divergent  after  its  removal,  but 
show  no  increase  of  divergence  by  the  contact;  and  the 
ball,  after  removal,  will  be  found  entirely  discharged. 
This  experiment  proves : — 

1.  That  the  induction  of  the  electrified   ball  has  re- 
pelled electricity  from  the  inner  to  the  outer  surface  of 
the  pail  if  the   charge  was  positive,  or  attracted  elec- 
tricity from  the  outer  to  the  inner  surface  if  the  charge 
was  negative ;  in  either  case  producing  a  divergence  of 
the  leaves. 

2.  It  proves  that  induction  increases  as  the  ball  de- 
scends, shown  by  the  increasing  divergence  of  the  leaves, 
till  all  the  lines  of  force,  which  can  be  included  within 
the  pail,  are  cut  by  its  surface,  after  which  there  is  no 
further  increase  of  divergence. 

3.  It   proves    that    there   is   no  permanent  effect  if 
there  is  no  contact ;  since  the  leaves  converge  when  the 
ball  is  removed. 

4.  It  proves  that  the  induced  charge  on  the  pail  is 
exactly  equal  to  the  charge  on  the  ball,  since  no  increase 
of  divergence  occurs  from  contact,  although  the  entire 
charge  has  been  communicated  to  the  pail,  as  shown  by 
the  ball  having  lost  its  charge.     But  this  can  be  strictly 
true  only  when  all  the  lines  of  force  are  cut  by  the  pail ; 
but  since  some  of  the  nearly  vertical  lines  must  escape, 
no  matter  how  deep  the  ball  descends,  there  must  be  a 
slight  increase  of  divergence  by  contact,  though  it  may 
not  be  perceptible. 

If  a  charge  be  given  to  the  pail  and  the  ball  be  low- 
ered into  it  by  a  wire  held  in  the  hand,  the  divergence 
of  the  leaves,  caused  by  the  charge  on  the  pail,  will  be 
perceptibly  reduced  as  the  ball  descends. 

This  proves  that  the  inner  surface  of  a  hollow  con- 


ELECTRIC  DISTRIBUTION  AND  CONDENSATION.         63 

ductor  can  be  charged  by  induction.  The  charge  on 
the  pail,  if  positive,  repels  electricity  from  the  ball, 
through  the  wire  and  hand,  to  the  earth ;  or,  if  nega- 
tive, attracts  electricity  from  the  earth;  and  in  either 
case,  a  certain  degree  of  equilibrium  follows,  causing  a 
corresponding  convergence  of  the  leaves. 

Entire  convergence  cannot  be  produced,  since  only 
a  small  portion  of  the  lines  of  force  from  the  pail  are 
cut  by  the  ball ;  while,  in  the  former  experiment,  nearly 
all  those  from  the  ball  were  cut  by  the  pail.  For  this 
reason  a  large  ball  is  best  for  the  second  experiment 
and  a  small  one  for  the  first. 

If  the  ball,  in  the  second  experiment,  is  lowered  by  a 
silk  cord  instead  of  a  wire,  there  is  no  perceptible  effect 
on  the  leaves,  since  induction  cannot  increase  nor  dimin- 
ish the  electricity  of  the  ball  when  there  is  no  earth 
connection. 

COMBINATION  OF  PAILS. — The  following  experiment 
was  made  by  Faraday  with  a  combination  of  hollow 
conductors : — 

Let  four  pails  of  different  sizes  be  placed  on  an  insu- 
lated support,  and  arranged  one  within  the  other  as 
shown  in  Fig.  12 :  and  let  them  be  insulated  from  each 
other  at  bottom  by  cakes  of  paraffin,  or  any  other  good 
insulator,  placed  between  them.  Let  silk  cords  be 
attached  to  the  three  inner  ones,  and  the  outer  one  be 
connected  with  an  electroscope. 

On  lowering  the  charged  ball  into  the  innermost  one, 
the  leaves  diverge  as  in  the  first  experiment ;  contact 
between  the  ball  and  pail  producing  no  increase  of 
divergence,  and  the  ball  is  then  found  to  be  discharged, 
as  before ;  which  proves  that  the  interposition  of  the 
insulated  pails,  2  and  3,  has  not  affected  the  induction. 


64 


ELEMENTS  OF  STATIC  ELECTRICITY. 


Now  let  pail  No.  4  be  lifted  out  by  the  silk  cord,  and 
the  leaves  will  converge,  and  diverge  again  when  it  is 
replaced,  showing  that  the  charge  on  the  ball  was  trans- 
ferred to  it. 


Fig.  12— Combination  of  Pails. 

Let  a  connection  be  now  made  by  pieces  of  copper 
wire,  let  down  by  silk  threads,  between  each  of  the 
pails  successively,  beginning  with  4  and  3,  till  all  four 
are  in  electric  connection,  and  let  the  effect  on  the 
leaves  be  observed  as  each  connection  is  made.  The 
results  will  be  found  the  same  as  in  the  first  experiment, 


ELECTRIC  DISTRIBUTION  AND  CONDENSATION.         65 

when  but  one  pail  was  used:  which  proves  that  the 
interposition  of  interior  surfaces  has  no  effect  on  induc- 
tion ;  nor  can  it  prevent  the  entire  charge  from  going 
to  the  outside  surface  when  the  four  pails  are  in  electric 
connection;  for  if  the  three  inner  pails  be  now  removed, 
they  will  be  found  to  have  lost  their  charge;  but  there 
will  be  no  change  in  the  divergence  of  the  leaves. 

This  experiment  is  an  actual  demonstration  of  what 
has  already  been  stated,  that  the  interior  of  a  solid  con- 
ductor, or  the  shell  of  a  hollow  conductor,  may  be 
regarded  as  composed  of  an  infinite  number  of  equipo- 
tential  shells  or  surfaces,  from  which  a  charge  of  elec- 
tricity must  always  pass  to  the  outside  surface. 

FARADAY'S  HOLLOW  CUBE. — A  most  remarkable  ex- 
periment in  this  connection  was  made  by  Faraday  with 
a  hollow  cube  of  wood,  measuring  twelve  feet  each  way, 
covered  with  tin-foil,  insulated  and  charged  by  a  power- 
ful electric  machine. 

He  says :  "  I  went  into  this  cube  and  lived  in  it, 
using  lighted  candles,  electrometers,  and  all  other  tests 
of  electrical  states.  I  could  not  find  the  least  influence 
upon  them,  or  indication  of  anything  particular  given 
by  them,  though  all  the  time  the  outside  of  the  cube 
was  powerfully  charged,  and  large  sparks  and  brushes 
were  darting  off  from  every  part  of  its  outer  surface." 

This  experiment  verifies  the  statement  made  on  page 
12  in  regard  to  zero  potential ;  showing  that  however 
strong  the  electrification, no  indications  of  electric  action 
are  perceptible  within  a  space  where  there  is  perfect 
equilibrium.  So  that  even  if  the  whole  earth  were  as 
powerfully  charged,  in  proportion  to  its  size,  as  Fara- 
day's cube,  we,  who  live  on  it,  could  perceive  no  electric 
action,  if  the  charge  were  as  uniform  as  on  the  cube. 


66  ELEMENTS  OF  STATIC  ELECTRICITY. 

But  if  it  be  objected  that  the  case  is  not  parallel,  see- 
ing that  we  live  on  the  surface,  it  must  be  remembered 
that  we  have  an  atmosphere  above  us  which  is  a  part  of 
the  earth's  matter;  so  that,  although  we  live  on  the 
solid  surface,  we  do  not  live  on  the  outer  surface :  and 
the  surface  on  which  we  live  is  practically  equipoten- 
tial  over  limited  areas. 

Faraday,  evidently,  might  have  generated  electricity 
with  insulated  instruments,  inside  the  cube,  and  con- 
densed it  on  insulated  conductors,  without  either  dis- 
turbing the  electric  conditions  by  which  he  was  sur- 
rounded, or  being  prevented  by  them :  just  as  we  do 
without  disturbing  the  earth's  electricity,  or  being  pre- 
vented by  it.  But  any  connection  by  a  conductor, 
between  his  instruments  and  the  cube,  would  have 
caused  the  charge  to  disappear;  just  as  a  similar  con- 
nection with  the  earth  produces  the  same  result. 

THICKNESS  OF  ELECTRIFIED  SURFACE. — The  idea 
of  surface  condensation  implies  that  an  electrified  sur- 
face must  be  something  more  than  a  mere  superficies. 
It  must  have  a  certain  degree  of  thickness,  the  elec- 
tricity penetrating  the  conductor  and  surrounding  air 
to  a  certain  depth,  in  proportion  to  the  resistance  of  the 
air,  and  the  attraction  or  repulsion  of  the  charge  on 
the  conductor.  Hence  the  amount  of  static  charge 
which  may  be  condensed  on  a  conductor,  per  unit  of 
surface,  depends  on  the  resistance  of  the  air. 

CONVECTION. — It  has  already  been  shown  that  dry 
air  is  one  of  the  best  insulators ;  but,  since  it  is  a  fluid, 
its  resistance  cannot  be  so  great  as  that  of  a  solid  of  the 
same  insulating  power;  for  the  air  molecules,  in  contact 
with  an  electrified  surface,  becoming  charged,  fly  off 
under  the  influence  of  repulsion  and  induction,  while 


ELECTRIC  DISTRIBUTION  AND  CONDENSATION.        67 

those  farther  out  rush  in  to  take  their  place ;  creating 
air  currents  around  the  conductor,  by  which  its  elec- 
tricity is  gradually  dissipated.  The  removal  of  electric- 
ity by  the  air  in  this  way  is  called  convection. 

VARIATION  OF  CHARGE. — Since  the  insulating  power 
of  the  air  varies  greatly  with  its  humidity  and  tempera- 
ture, and  since  its  electric  potential  is  also  variable,  the 
charge  which  may  be  condensed  on  a  conductor  will 
vary  in  like  proportion  ;  dry,  cold  air  being  much  more 
favorable  to  the  condensation  of  a  high  charge  than 
damp,  warm  air;  and  air  at  a  high  electric  potential 
than  air  at  a  low  potential. 

Analogous  to  this  is  the  influence  of  atmospheric 
pressure  on  steam ;  the  temperature  varying  with  the 
pressure  under  which  it  is  generated.  Here  pressure 
constitutes  resistance,  while  in  the  case  under  consider- 
ation the  resistance  is  due  to  the  causes  mentioned. 

Equal  electric  condensation  on  every  part  of  the  sur- 
face is  never  practically  true ;  as  the  induction  of  sur- 
roundings varies,  and  form,  as  will  be  shown  hereafter, 
has  an  important  influence.  It  could  only  be  true  of 
an  insulated  sphere,  surrounded  by  a  homogeneous 
medium,  and  removed  from  all  other  influences. 

INFLUENCE  OF  FORM. — It  has  already  been  stated 
that  form  exercises  an  important  influence  on  the 
amount  of  static  charge  which  may  be  condensed  on  a 
conductor  ;  and  that  a  charge  on  an  insulated  sphere 
is  equally  distributed  over  its  surface,  when  the  sur- 
rounding induction  is  equal :  also  that  the  air,  by  its 
insulation,  retains  this  charge  on  the  surface,  and 
by  its  convection  gradually  removes  it.  It  is  evident 
also  that  these  forces  act  at  equal  distances  from  the 
center. 


68 


ELEMENTS  OF  STATIC  ELECTRICITY. 


Fig.  13— Spheres  in  Contact. 


ELECTRIFIED  SPHERES. — Let  two  insulated  metal 
spheres,  of  equal  size  and  similarly  charged,  be  placed 
in  contact,  as  represented  in  Fig.  13.  It  is  evident  that 
either  .of  them,  separately,  would  fulfill  the  conditions 

just  named;  but 
when  placed  in  con- 
tact, they  must  be  re- 
garded as  one  mass, 
having  its  center  at 
the  point  of  contact ; 
the  electric  distribu- 
tion being  the  same 
on  each. 

Hence  the  forces 
of  induction  and  re- 
pulsion which  before  acted  to  remove  electricity  from 
the  center  of  the  single  sphere  to  the  parts  most  remote 
from  it  —  that  is,  to  the  surface — now  act  in  the  same 
manner,  to  remove  electricity  from  this  new  center  to 
those  parts  of  the  mass  most  remote,  that  is,  to  the 
points  A  and  B,  and  the  surfaces  surrounding  them. 

There  must  also  be  a  certain  amount  of  electricity 
distributed  over  the  entire  surface  of  each  sphere  ;  and 
there  must  be  repulsion  between  the  surfaces  adjacent 
to  the  point  of  contact :  so  that  the  charge  will  be  zero 
at  this  point,  and  increase  each  way  toward  A  and  B. 

This  may  be  demonstrated  by  touching  the  points 
A,  B,  and  C  with  the  proof  plane,  and,  after  each  con- 
tact, bringing  it  near  the  disc  of  the  electroscope ; 
taking  care  to  discharge  it  with  the  finger  before  mak- 
ing the  next  test. 

It  will  be  found  that  the  central  point  shows  scarcely 
a  trace  of  electricity,  while  the  points  A  and  B  are 


ELECTRIC  DISTRIBUTION  AND  CONDENSATION.        69 

strongly  electrified.  The  same  test,  applied  to  inter- 
mediate points,  shows  the  charge  on  them  to  be  in  pro- 
portion to  their  distance  from  the  central  point. 

ELECTRIFIED  CYLINDER.— Instead  of  the  two  spheres, 
we  may  substitute  an  insulated  metal  cylinder,  with 
hemispherical  ends,  provided  with  pith  -  ball  •  electro- 
scopes at  the  ends  and  center,  as  represented  in  Fig.  14. 

A  light  charge  of  electricity   G^^ 
on  the  cylinder  will  cause  the 
balls  at  the  ends  to  diverge  in 
opposite  directions,    while    the 
central    ball    will   remain    un- 
moved, or  but  slightly  affected  ; 
showing  that  the  principal  part 
of  the  charge  is   condensed  on 
the    ends,    and   that   induction 
and  repulsion  are  operating  to 
remove  electricity  to  the  points    Fig'  ^- 
farthest  from  the   center,  as  shown  by  the  position  of 
the  balls  at  the  ends. 

If  a  sphere  be  made  to  oscillate  near  one  of  the  balls, 
at  right  angles  to  the  length  of  the  cylinder,  the  effect 
of  induction  will  be  shown  by  the  ball  following  the 
movement  of  the  sphere. 

INFLUENCE  OF  POINTS. — If  a  cylinder  having  cone- 
shaped  ends  be  substituted  for  the  one  with  hemispher- 
ical ends,  dissipation  of  the  charge,  instead  of  condensa- 
tion, will  occur.  For,  on  the  hemispherical  ends,  the 
charge  is  retained  by  the  resistance  of  the  air  on  the 
surface ;  but  the  cone-shaped  ends  terminate  in  points 
which  have  no  surface,  hence  there  can  be  no  resistance. 

But  if  resistance  is  removed,  even  from  a  single  point, 
it  is  evident  that  the  entire  charge  must  pass  off  through 


70  ELEMENTS  OF  STATIC  ELECTRICITY. 

that  point ;  since  the  removal  of  electricity  from  any 
point  on  a  surface  creates  a  difference  of  potential  be- 
tween that  and  surrounding  points,  producing  an  elec- 
tric movement  in  the  direction  of  the  point  of  no 
.resistance,  which  must  extend  to  every  part  of  the  sur- 
face, and  continue  till  equilibrium  is  restored. 

Instead  of  the  cylinder  with  cone-shaped  ends,  we 
may  use  one  with  needles  attached  to  the  ends,  as  repre- 
sented in  Fig.  15.  A  wooden  cylinder  covered  with 
tin-foil  can  easily  be  changed  in  this  way. 

It  will  be  impossible  to 
charge  such  a  cylinder,  even  if 
only  a  single  needle  be  at- 
tached to  any  part  of  the  sur- 
face. A  projecting  angle  on 
any  part  of  a  conductor  will 
tend  to  produce  the  same  re- 
sult. 

Effects  somewhat  analogous 
to   these  may  be  obtained  by 
Wi7h  Points    Dipping  into  water  a  spherical 
Attached.  body,  and  also  a  sharp-pointed 

spike  having  the  same  amount  of  surface.  On  lifting 
out  the  spherical  body,  water  will  adhere  to  it,  and  col- 
lect in  a  large  drop  at  the  lowest  part ;  being  held  there 
by  adhesion  and  atmospheric  pressure.  But  if  the 
spike  be  lifted  out,  point  downwards,  the  water  will 
drop  off  when  it  reaches  the  lowest  point,  there  being 
no  surface  there  on  which  it  can  be  retained  by  those 
forces. 

ELECTRIFIED  SPHEROID. — If  a  metal  sphere  be  flat- 
tened at  the  poles  till  it  assumes  the  form  of  an  oblate 
spheroid,  as  shown  at  A,  Fig.  16,  the  face  of  a  cross- 


ELECTRIC  DISTRIBUTION  AND  CONDENSATION. 


71 


j 
V 


j 


section  through  the  poles,  as  shown  at  j5,  will  have  the 

same  form  as  a  cylinder  with  hemispherical  ends.     And 

since  it  has  been  shown  that  a  charge  of  electricity  on 

such  a  cylinder  is  condensed 

on  the  ends,  it  is  evident  that 

a  charge  on  such  a  spheroid 

will,  in  like  manner,   be   con- 

densed on  its  outer  edge. 
ELECTRIFIED   Disc.  —  If  a 

flat    metal    disc,   with   a    thin 

edge,  be  electrified,  the  charge 

will  go  to  the  outer  edge,  as  in 

the  last  case.     But  resistance, 

being  in  proportion  to  surface, 

is  very  small  on  such  an  edge,     Fig'  16-Electrified  Spheroid. 

and  the  charge  is  rapidly  dissipated.      Hence  such  a 

disc,  when  constructed  for  the  purpose  of  condensing 

electricity  on  it,  should  be  pro- 
vided  with  a  round  rim,  which 
may  be  called  a  resistance  rim. 

If  it  be  insulated,  and  there 
be  placed  on  its  opposite  sides, 
near  the  edge,  two  little  metal 
stands  with  pointed  stems,  on 
which  are  balanced  light  metal 
pointers,  having  arms  of  tin- 

Fig.  17-ElectrifledDisc.        equal  ^^  &g  shown   jn   Fig> 

17,  a  charge  of  electricity  given  to  it  will  cause  the 
pointers  to  arrange  themselves  in  the  direction  of  the 
radii,  showing  that  the  electric  force  is  from  the  center 
outward. 


CHAPTER  VI, 

ACCUMULATORS. 

THE  CHARGED  PANE. — The  electric  charge  which 
be  condensed  on  the  surface  of  an  insulated  con- 
ductor is  comparatively  small,  when  such  a  conductor 
is  remote  from  inductive  influence. 

But  when  another  conductor,  having  a  connection 
with  the  earth,  is  placed  in  its  immediate  vicinity, 
the  charge  may  be  greatly  increased. 

To  prove  this,  let  a  sheet  of  good  insulating  glass, 

varnished   with    shellac,    be 
coated  on  opposite  sides  with 
tin-foil,  to  within  about  two 
inches  of  its,  edge,  and  placed 
on  an  insulating  support,  as 
shown  in  Fig.  18.     A  small 
charge  can  be  given  to  the 
Fig.  18-The  Charged  Pane,      tin-foil,  on  the  upper  surface, 
which  will  be  indicated  by  sparks  passing  between  it 
and  the  body  from  which  the  charge  is  given.     But  the 
limit  is  soon  reached,  and  no  more  sparks  will  pass. 

Now  let  the  lower  surface  be  connected  with  the  earth 
by  a  strip  of  tin-foil,  and  sparks  will  again  pass  freely 
between  the  charging  body  and  the  upper  surface,  till 
a  charge  greatly  in  excess  of  the  former  is  given. 

If  the  tin-foil  strip  be  suspended  with  its  lower  end 
near  a  conductor,  as  shown,  sparks  will  pass  between 


AC  CUM  ULA  TORS.  1 3 


it  and  the  conductor,  simultaneously  with  the  sparks 
on  the  upper  surface ;  indicating  that  each  surface  re- 
ceives the  same  amount  of  charge. 

But  the  potential  on  opposite  surfaces  will  be  oppo- 
site. If  the  upper  surface  acquires  positive  potential, 
by  an  increase  of  electricity,  the  same  amount  will  be 
repelled  from  the  lower  surface,  making  it  negative. 
But  if  the  upper  becomes  negative  by  a  decrease, 
electricity,  to  the  same  amount,  will  be  attracted  to  the 
lower  surface,  making  it  positive. 

To  prove  that  these  charges  are  equal,  let  the  tin- 
foil strip  be  removed 
after  the  plate  has  been 
charged  ;  and  a  wire, 
held  by  a  piece  of 
india  -  rubber  tube,  to 
insulate  it,  be  bent  so 
that  its  ends  come  into 
contact  with  the  oppo- 
site surfaces,  as  shown 

Fig.  19— The.  Pane  Discharged. 

in  Fig.  19 :    a  flash  and 

report  will  follow,  and  both  surfaces,  after  the  wire 
has  remained  in  contact  for  a  few  moments,  will  be 
found  completely  discharged. 

Now,  since  the  removal  of  the  strip  produced  com- 
plete insulation,  perfect  equilibrium  could  occur  only 
by  the  positive  of  one  surface  being  exactly  equal  to 
the  negative  of  the  other. 

Since  induction  varies  inversely  as  the  square  of 
the  distance  (page  47),  it  is  evident  that,  if  this  factor 
alone  is  considered,  the  amount  of  charge  which  can  be 
given  will  be  in  the  inverse  ratio  of  the  thickness  of 
the  glass,  and  hence  greater  on  thin  than  on  thick 


74  ELEMENTS  OF  STATIC  ELECTRICITY. 

glass.  But  since  the  resistance  of  glass  is  in  the 
direct  ratio  of  its  thickness,  when  the  specific  induct- 
ive capacity  is  the  same,  this  factor  also  must  be 
considered. 

Hence,  in  the  construction  of  instruments  involving 
these  principles,  if  great  sensitiveness  and  a  low  poten- 
tial is  desired,  the  glass,  or  other  dielectric,  should  be 
thin  :  but  if  the  highest  attainable  potential  is  desired, 
there  should  be  sufficient  thickness  to  resist  fracture 
or  puncture. 

The  uncoated  margin  must  also  be  wide  enough  to 
m^ike  the  resistance  there  equal  to  that  of  the  thickness  ; 
a  small  fraction  of  an  inch  in  thickness  having  a  re- 
sistance equal  to  that  of  several  inches  of  surface. 

No  definite  rules  can  be  given,  as  the  resistance  of  va- 
rious kinds  of  glass,  and  other  dielectrics,  varies  greatly, 
as  well  as  the  cases  in  which  they  may  be  required. 

As  the  positive  and  negative  on  opposite  surfaces  are 
equal,  it  is  impossible  for  a  change  of  potential  to  occur 
on  either  surface  without  a  corresponding  change  on 
the  opposite  surface.  Hence  a  conductor  brought  into 
contact  with  either  surface  alone  will  not  change  its 
potential,  unless  directly  or  indirectly  connected  with 
the  opposite  surface.  Hence  the  charge  on  each  surface 
is  said  to  be  bound  by  the  opposite  charge. 

The  convection  and  conduction  of  the  air,  so  far 
as  it  can  act  equally  on  both  surfaces,  will  in  time  re- 
store equilibrium.  It  may  also  be  restored  by  the  oscil- 
lation of  a  solid  body,  as  a  pith  ball,  suspended  between 
conductors  connected  with  both  surfaces  ;  or,  by  direct 
connection  through  a  conductor,  as  already  explained. 

Instruments  constructed  for  accumulating  electricity 
in  this  way  are  called  accumulators,  or  condensers. 


ACCUMULATORS.  75 

THE  LEYDEN  JAR. — The  first  discovery  of  an  accu- 
mulator was  made  by  Kleist,  a  clergyman  of  Cammin, 
in  Pommerania,  who  stated  in  a  letter  to  Dr.  Lieber- 
kiihn,  of  Berlin,  Nov.  4, 1745,  that  by  pouring  a  little 
mercury,  "  spirits,"  or  water,  into  a  phial  and  con- 
necting it  with  a  nail  through  the  cork,  he  could 
electrify  it  through  the  nail,  ignite  "spirits  of  wine" 
with  it,  and  receive  a  shock  by  touching  the  nail  with 
his  finger. 

The  same  discovery  was  made  in  the  following  year 
in  Leyden,  by  Cuneus,  a  pupil  of  Musschenbroek,  who 
electrified  some  water  in  a  flask,  which  he  held  in  his 
hand,  by  bringing  it  into  contact  with  a  chain  from  the 
conductor  of  an  electric  machine.  On  attempting  to 
remove  the  chain  with  his  other  hand,  he  received  an 
electric  shock  which  so  frightened  him  that  he  dropped 
the  flask.  Musschenbroek,  having  tried  the  experiment, 
said  he  would  not  take  a  second  shock  for  the  crown 
of  France. 

The  discovery  created  great  excitement,  and  led  to 
the  construction  of  improved  instruments,  to  which 
the  name  "Leyden  jar"  was  given. 

The  water  in  this  instance  constituted  the  inside 
coating,  the  hand  the  outside  coating ;  and,  when  the 
other  hand  touched  the  chain,  both  surfaces  were  con- 
nected by  a  conductor,  and  a  discharge  followed,  which 
produced  the  shock. 

Fig.  20  represents  the  Leyden  jar  as  it  is  usually 
constructed.  The  essential  elements  are  two  conduct- 
ing surfaces  separated  by  a  dielectric  ;  but,  for  conven- 
ience in  charging  and  discharging,  a  wooden  cap  is 
fitted  to  it,  through  which  passes  a  brass  rod,  terminat- 
ing in  a  ball  above,  and  to  its  lower  end  is  attached  a 


76 


ELEMENTS   OF  STATIC  ELECTRICITY. 


light  Spring,  or  a  chain,  which  comes  into  contact  with 
the  inside  coating. 

Tin-foil  is  the  usual  coating,  and  is  put  on  with  paste, 
covering  both  surfaces  equally  to  within  about  three 
inches  of  the  top.  Light  sheet  brass  makes  a  more 
substantial  outside  coating,  and  does  not  require  at- 
tachment to  the  surface. 
It  can  also  be  used  for 
the  inside  coating,  when 
the  mouth  is  the  full 
width  of  the  jar  and  the 
sides  are  straight.  Sul- 
phuric acid  is  also  some- 
times used  for  the  inside 
coating  of  jars  designed 
fur  special  purposes. 

An  instrument  called 
a  discharger  is  also  repre- 
sented at  A,  in  Fig.  20. 
It  consists  of  a  curved 
brass  rod,  terminating 
in  balls,  and  having  an 
insulating  handle,  of 
ebonite  or  glass,  at- 

Fig.  20-Leyden  Jar  and  Discharger.          tached  to  its  Center.     It 

is  sometimes  jointed  at  the  center,  and  furnished  with 
two  handles,  as  represented  at  B,  Fig.  20.  Its  use  is  the 
same  as  that  of  the  bent  wire  already  described. 

The  Leyden  jar  can  be  made  of  any  insulating 
material  capable  of  being  molded  into  the  proper  form; 
but  glass  seems  to  be  the  only  substance  capable  of 
resisting  the  enormous  strain  to  which  the  dielectric  is 
subjected  under  a  full  charge. 


A  CCVMULA  TORS.  1 1 

Glass  suitable  for  the  purpose  must  be  free  from  any 
substance  which  makes  it  a  partial  conductor.  Hence 
such  glass  as  is  commonly  used  for  fruit  jars,  candy 
jars,  and  druggist's  bottles  cannot  be  used,  since  it  con- 
tains metallic  substances. 

Glass  of  a  bright  green  color,  free  from  bluish  tint, 
also  the  kind  known  as  "hard  flint,"  makes  the  best 
Ley  den  jars 

The  Leyden  jar  is  charged  by  an  electric  machine ; 
its  inner  coating  being  connected  with  the  machine, 
and  its  outer  coating  with  the  earth,  or  with  the  op- 
posite electrode  of  the  machine ;  though  it  is  not 
material  which  coating  is  connected  with  the  machine, 
except  as  a  matter  of  convenience.  The  jar  may 
be  insulated,  and  the  charge  given  to  the  outer 
coating,  if  the  inner  coating  is  connected  with  the 
earth. 

It  is  also  immaterial  whether  the  charge  given  is 
positive  or  negative,  as  the  opposite  charge  will  always 
be  induced  on  the  opposite  surface  ;  electricity  being 
repelled  to  the  earth  when  a  positive  charge  is  given, 
or  attracted  from  the  earth  when  negative  is  given. 

The  electromotive  force  (E.  M.  F.)  of  a  jar  is  equal 
to  the  difference  of  potential  between  its  inner  and 
outer  coatings. 

CHARGE  BY  CASCADE. — The  method  of  charge  by 
cascade,  first  proposed  by  Franklin,  is  as  follows :  Let 
a  number  of  jars  of  equal  size,  as  A,  B,  C,  Z>,  be 
arranged  as  represented  in  Fig.  21 ;  the  outer  coating 
of  each,  commencing  with  A^  being  connected  with  the 
inner  coating  of  the  one  next  to  it ;  D  having  its  outer 
coating  connected  with  the  earth,  and  A  having  its 
inner  coating  connected  with  the  machine.  And  let  A, 


78  ELEMENTS  OF  STATIC  ELECTRICITY. 

B,  and  C  be  well  insulated  on  cakes  of  paraffin  or  some 
equally  good  insulator. 

A  positive  charge  given  to  the  inner  coating  of  A 
will  induce  negative  on  its  outer  coating,  by  repelling 
the  same  amount  of  electricity;  and  this  repelled 
charge  must  go  to  the  inside  of  B,  since  it  has  no  other 
outlet.  Hence  the  inner  coating  of  B  will  be  positively 
charged,  and  electricit}^  will,  in  like  manner,  be  repelled 
from  its  outer  coating  to  the  inner  of  C.  Hence  the 
charge  of  each  jar  in  the  series  will  be  similar  to  that 
of  A ;  electricity  from  the  outer  coating  of  D  being 
repelled  to  the  earth. 


Fig.  21— Jars  in  Cascade. 

As  the  energy  expended  is  distributed  among  four  jars, 
it  is  evident  that  the  charge  of  each  must  be  much  less 
than  if  the  same  amount  had  been  expended  in  charging 
one  jar :  since  the  energy  accumulated  cannot  exceed 
the  energy  expended.  But,  as  the  charge  is  in  the 
inverse  ratio  of  the  thickness  of  the  glass,  the  resist- 
ance from  this  source  must  increase  from  A  to  7>,  in 
proportion  to  the  number  of  thicknesses  interposed : 
and  the  charge  must  vary  in  the  same  ratio ;  the  neg- 
ative being  greatest  on  the  outer  coating  of  A,  where 
only  one  thickness  is  interposed,  and  least  on  the  outer 
coating  of  D,  where  four  thicknesses  are  interposed ; 


ACCUMULATORS.  79 


C 

the  positive  on  the  inner  coatings  varying  in  the  same 

ratio.  The  same  variation  must  also  occur  in  the 
resistance  of  the  connectors,  and  produce  a  similar 
effect,  in  a  limited  degree  ;  the  resistance  of  a  conductor 
being  directly  as  its  length. 

If  the  charge  given  to  the  inner  coating  of  A  be 
negative,  the  electric  movement  is  reversed;  all  the 
inner  coatings  becoming  negative,  and  the  outer  pos- 
itive ;  electricity  being  attracted  from  the  earth  to  the 
outer  coating  of  D. 

The  insulations  and  connections  should  receive  care- 
ful attention,  so  as  to  prevent  loss  by  leakage  ;  which 
will  inevitably  occur  if  the  insulation  is  imperfect,  or 
if  the  connectors  have  points,  sharp  edges,  or  projecting 
corners. 

After  the  charge  is  given,  the  jars  should  be  sep- 
arated, placed  in  connection  with  the  earth,  and  each 
discharged  separately.  A  single  jar,  charged  to  the 
same  amount,  should  then  be  discharged,  and  the 
results  -compared. 

This  method  will  indicate,  roughly,  the  amount  of 
charge  of  each  jar  ;  but  the  electrometer,  to  be  de- 
scribed hereafter,  will  give  more  accurate  results. 

THE  LEYDEN  BATTERY.  —  When  a  number  of  jars 
have  their  inner  coatings  joined  by  conductors,  and 
also  their  outer  coatings  in  like  manner,  the  combi- 
nation is  called  a  Leyden  battery. 

A  convenient  form  of  such  a  battery  is  represented 
by  Fig.  22,  in  which  connectors  between  the  inner 
coating  radiate  from  a  central  jar.  The  outer  coatings 
are  made  of  sheet  brass,  nickel-plated,  and  screwed  to  a 
wooden  base,  their  connections  being  made  with  copper 
wires  attached  to  the  points  of  the  screws  underneath. 


80  ELEMENTS  OF  STATIC  ELECTRICITY. 

This  construction  for  the  outer  coatings  makes  them 
durable,  gives  the  jars  a  firm  attachment,  and  adds 
greatly  to  the  neatness  and  beauty  of  the  instrument. 

The  E.  M.  F.  of  a  Leyden  battery  is  the  same  as  that 
of  a  single  jar  having  the  same  amount  of  coated  sur- 
face. There  can  be  no  increase  of  intensity  from  any 
special  arrangement  of  the  jars,  as  such  a  battery  is 
merely  an  accumulator,  and  not  a  generator "of  electric- 


Fig.  22— Leyden  Battery. 

ity.  But  when  great  E.  M.  F.  is  required  it  is  generally 
more  convenient  to  use  a  battery  than  a  single  jar  of 
equal  energy.  And,  in  case  of  fracture  from  an  over- 
charge, a  small  jar  can  be  replaced  at  less  expense  than 
a  larger  one. 

In  charging  or  discharging  a  battery,  it  is  immaterial 
which  jar  is  selected :  for  all  the  inner  coatings  being 
connected  together,  as  well  as  all  the  outer  coatings, 
each  is  practically  the  same  as  a  single  coating  of  equal 


ACCUMULATORS.  81 

size ;  and  connection  with  any  part  of  either  coating 
affects  the  whole  of  that  coating. 

DISCHARGE  THROUGH  A  BOOK.  —  The  discharge 
from  a  Leyden  jar  or  battery,  passed  through  a  card  or 
a  thin  book,  leaves  a  puncture,  with  a  burr  projecting 
from  each  surface. 


Fig.  23— Discharge  Through  a  Book. 

To  perform  this  experiment  successfully,  let  one 
knob  of  the  discharger  be  placed  in  contact  with  the 
outer  coating,  and  the  other  in  contact  with  the  book ; 
and  let  the  book,  held  by  its  edge,  with  the  knob 
against  it,  be  brought  quickly  into  contact  with  the 
knob  of  the  jar,  and  the  discharge  will  take  place  as 
shown  in  Fig.  23. 


82  ELEMENTS  OF  STATIC  ELECTRICITY. 

In  this  way  a  book  of  one  hundred  or  more  pages  may 
be  perforated. 

If  the  book  is  first  placed  in  contact  with  the  knob 
of  the  jar,  part  of  the  charge  will  escape  from  the  edges 
and  corners  of  the  leaves,  and  the  experiment  is  liable 
to  fail. 

The  burr  projecting  from  each  surface,  after  the  dis- 
charge through  a  book  or  card,  has  been  relied  on  as  a 
proof  of  the  dual  nature  of  electricity,  and  ascribed  to 
the  rush  of  positive  and  negative  in  opposite  directions. 
It  is  also  attributed  to  the  expansive  force  of  heat,  or  of 
gas,  generated  by  the  discharge. 

The  first  theory  cannot  be  accepted,  unless  we  have 
stronger  proof  of  the  dual  nature  of  electricity  than  is 
afforded  by  this  experiment.  And  the  second  also  fails ; 
since  in  the  case  of  a  discharge  through  a  book,  the 
leaves  may  be  held  so  loosely  as  to  allow  a  free  outlet 
for  expansion  from  heat  or  gas,  and  yet  the  burr  turns 
in  opposite  directions  from  a  point  near  the  center  of 
the  book,  and  becomes  more  prominent  when  the  leaves 
are  thus  held  than  when  they  are  compressed  ;  whereas, 
if  the  burr  were  due  to  the  expansive  force  of  confined 
heat  or  gas,  the  reverse  would  be  true. 

Since  these  theories  are  unsatisfactory,  let  us  en- 
deavor to  explain  this  phenomenon  in  accordance  with 
the  principles  which  we  have  been  considering. 

Let  a  jar  be  charged  on  its  inner  coating,  and 
discharged  through  a  book,  as  represented  in  Fig.  23. 
Suppose  the  charge  to  be  positive,  electric  movement 
being  from  higher  to  lower  potential,  it  would  be  from 
the  knob  of  the  jar  to  the  nearest  knob  of  the  dis- 
charger. The  entire  charge  of  the  inner  coating, 
passing  out  through  the  knob,  would  induce  a  high 


ACCUMULATORS.  83 

negative  potential  on  that  point,  on  the  nearest  surface 
of  the  book,  in  a  line  between  the  knobs  ;  repelling  the 
electricity  of  the  book  along  that  line  to  the  opposite 
surface,  which  would  thus  become  highly  positive. 

The  paper  being  a  very  imperfect  conductor,  the 
charges  thus  induced  do  not  spread  rapidly,  but  remain 
concentrated  for  a  moment  on  small  circular  spaces 
around  each  of  these  points ;  the  greatest  intensity 
being  at  the  centers.  Hence  there  is  a  powerful  at- 
traction between  the  knob  of  the  jar  and  this  negative 
point  on  the  surface  of  the  book  ;  and  also  between  the 
knob  of  the  discharger  and  the  positive  point  on  the 
other  surface ;  under  the  influence  of  which  the  paper 
on  each  surface  gives  way  and  bursts  outward  toward 
the  knobs ;  that  surface  next  the  knob  of  the  jar  being 
attracted,  and  that  next  the  knob  of  the  discharger 
repelled. 

As  each  outward  leaf  bursts,  the  next,  becoming 
then  the  outer  one,  bursts  also,  till  the  perforation  is 
complete  from  the  center  each  way.  All  of  which 
occurs  instantaneously. 

Meantime  the  electricity  from  the  knob  of  the  jar 
follows  up  this  inductive  effect  on  the  electricity  of  the 
book;  but  meeting  great  resistance  from  the  imper- 
fectly conducting  paper,  and  the  air  between  the  leaves, 
it  is  concentrated  on  each  leaf  successively ;  so  that  the 
inductive  force  is  constantly  in  advance  of  the  charge, 
the  leaves  and  layers  of  air  between  them  constituting 
the  dielectric. 

It  will  be  noticed,  then,  that  this  is  not  a  case  of 
energy  going  through  a  passive  medium,  but  of  energy 
acting  on  the  energy  of  that  medium,  causing  it  to  become 
active  and  perform  work. 


84  ELEMENTS  OF  STATIC  ELECTRICITY. 

It  should  also  be  noticed  that  when  the  leaves  are 
held  loosely,  the  thickness  of  the  air  dielectric  is  in- 
creased ;  each  layer  of  air  having  a  charged  surface  of 
partly  conducting  paper  on  each  side  of  it,  is  in  the 
position  of  the  coated  pane,  a  powerful  attraction 
between  the  surfaces  acting  across  it.  And  when  the 
paper  bursts  there  is  more  room  for  the  formation  of  a 
burr,  and  less  resistance  to  the  tearing  of  the  paper, 
which  accounts  for  the  increased  prominence  of  the 
burr. 

If  the  charge  of  the  jar  is  negative,  the  same  results 
occur  in  reverse  order. 

THE  RESIDUAL  CHARGE. — When  a  Leyden  jar  is 
discharged,  there  still  remains  a  slight  difference  of 
potential  between  the  coatings,  which  is  known  as  the 
residual  charge.  Hence,  a  small  discharge  can  be 
obtained  a  moment  after  the  first ;  and  this  also  leaves 
a  residual,  bearing  about  the  same  proportion  to  the 
second  discharge  as  the  second  to  the  first,  when  the 
same  length  of  time  elapses  between  them.  A  number 
of  successive  discharges  may  thus  be  obtained,  which 
constantly  decrease  in  amount  till  no  further  discharge 
is  perceptible.  But,  even  then,  it  is  not  probable  that 
perfect  equilibrium  is  restored. 

To  understand  this,  we  must  remember  that  even 
the  best  dielectric  is  a  partial  conductor:  and  that 
while  electric  movement  is  instantaneous  in  a  good 
conductor,  it  is  very  slow  in  a  non-conductor.  In  the 
Leyden  jar  we  have  a  combination  of  both — two  con- 
ductors separated  by  a  non-conductor.  And,  when  the 
charge  is  given,  every  part  of  each  coating  instantly 
becomes  electrified,  one  coating  positively  and  the  other 
negatively,  on  the  surfaces  next  the  glass. 


ACCUMULATORS.  '        85 

The  electricity,  on  the  positively  electrified  coating, 
slowly  penetrates  into  the  glass,  acting  inductively  on 
its  electricity,  which  it  repels  from  the  opposite  sur- 
face ;  and  producing,  probably,  a  temporary  strain  or 
distortion  of  its  structure. 

When  the  first  discharge  takes  place,  there 
is  a  relief  from  this  strain;  and,  as  the 
electrified  glass  slowly  returns  to  its  former 
state,  the  electricity  which  had  penetrated  it 
returns  to  the  conducting  surface. 

This  view  receives  confirmation  from  the 
fact  that  delay  increases  the  residual  charge, 
giving  time  for  the  electricity  to  come  out  of 
the  glass  and  accumulate :  while  it  has  the 
opposite  effect  on  the  primary  charge,  reduc- 
ing it  by  giving  time  for  dissipation. 

Tapping  the  jar  lightly  hastens  the  in- 
crease of  the  residual  charge,  the  vibratory 
motion  thus  given  to  the  glass  tending,  prob- 
ably, to  relieve  the  electric  strain. 

JAB    WITH    MOVABLE    COATINGS. — If  a  with  2 


Leyden  jar  be  constructed  Avith  any  rigid  Coatluss- 
metal,  as  sheet  brass,  for  both  coatings,  as  suggested  on 
page  76,  and  the  conducting  rod  be  attached  to  the 
inner  coating,  the  coatings  may  be  removed  and  re- 
placed at  pleasure,  as  represented  in  Fig.  24 :  and  we 
have  the  means  of  investigating  certain  phenomena  in 
regard  to  the  electrification  of  the  different  parts. 

Let  a  charge  be  given  to  such  a  jar,  and  the  coatings 
removed  carefully,  so  that  they  shall  not  be  connected 
by  a  conductor  during  removal:  they  may  now  be 
brought  into  contact  without  producing  any  electric 
effect ;  and  the  jar  also  may  be  handled  with  a  like 


86  ELEMENTS  OF  STATIC  ELECTRICITY. 

result :  but,  on  replacing  the  coatings,  a  full  discharge  can 
be  obtained,  the  same  as  if  they  had  not  been  removed. 

But  if,  while  the  coatings  are  removed,  the  jar  be 
examined  by  touching  both  surfaces  with  the  finger  and 
thumb,  or  a  small  discharger,  made  with  a  bent  wire, 
at  any  point  below  a  line  marking  the  position  of  the 
upper  edges  of  the  coatings,  a  discharge  can  be  obtained 
from  that  point.  In  this  way  a  number  of  small  dis- 
charges can  be  had  from  various  points,  but  no  general 
discharge. 

This  proves  that  the  charge  remains  on  the  glass,  while 
the  coatings  are  removed  ;  but  that  the  resistance  of  the 
glass  prevents  a  general  discharge.  But  it  cannot  be 
accepted  as  proof  that  the  charge  is  confined  to  the 
glass,  when  the  coatings  are  in  contact  with  it ;  unless 
it  can  be  shown  that  the  charge  remains  on  the  glass 
after  the  removal  of  both  coatings  at  precisely  the  same 
instant ;  which  could  not  be  done  with  the  care  neces- 
sary for  so  delicate  an  experiment.  But  when  the 
coatings  are  removed  separately,  the  charge  must  be 
transferred  to  the  glass  during  the  removal  of  each : 
since  it  is  impossible  to  produce  any  change  of  poten- 
tial on  either  surface,  unless  a  corresponding  change 
is  produced,  at  the  same  instant,  on  the  opposite  surface; 
each  being  bound  by  the  opposite. 

VARIOUS  EFFECTS  OF  THE  DISCHARGE. — The  dis- 
charge of  a  Ley  den  jar  of  moderate  size  is  sufficient  to 
explode  gunpowder,  and  to  ignite  various  substances; 
as  phosphorus,  powdered  resin,  sulphuric  ether,  and 
alcohol;  while  that  of  a  large  Leyden  battery  fuses 
wires,  magnetizes  steel,  'and  destroys  animal  life. 

With  a  battery  of  550  square  feet  of  coated  surface, 
large  steel  bars  have  been  magnetized,  iron  wires,  T£o 


ACCUMULATORS.  87 

of  an  inch  in  diameter,  and  25  feet  long,  melted  into 
globules;  and  tin  wires,  -fa  of  an  inch  in  diameter,  and 
8  inches  long,  dissipated  in  smoke. 

Tyndall  accidentally  received  a  charge  from  a  Ley- 
den  battery  of  "  fifteen  large  jars "  during  a  lecture, 
and  describes  his  experience  as  follows :  "  For  a  sensi- 
ble interval  life  was  absolutely  blotted  out,  but  there 
was  no  trace  of  pain.  After  a  little  time  consciousness 
returned;  I  saw  confusedly  both  the  audience  and  the  ap- 
paratus. But  though  the  intellectual  consciousness  of  my 
position  returned  with  exceeding  rapidity,  it  was  not  so 
with  the  optical  consciousness.  For  my  body  presented  to 
my  eyes  the  appearance  of  a  number  of  separate  pieces. 
The  arms,  for  example,  were  detached  from  the  trunk 
and  suspended  in  the  air.  In  fact,  memory  and  the 
power  of  reasoning  appeared  to  be  complete  long  before 
the  restoration  of  the  optic  nerve  to  healthy  action." 

Gunpowder  cannot  be  exploded  by  the  ordinary 
discharge;  the  only  effect  of  which  is  to  scatter  it.  But 
when  the  discharge  is  retarded,  by  introducing  into  the 
circuit  an  imperfect  conductor,  as  a  wet  string,  it 
explodes  readily.  By  this  method  also  gun-cotton, 
phosphorus,  and  other  highly  inflammable  substances 
may  be  ignited. 

For  such  experiments  the  universal  discharger,  rep- 
resented by  Fig.  25,  is  convenient.  It  is  constructed 
with  a  base,  in  the  center  of  which,  mounted  on  a  stem, 
is  a  small  circular  tablet  of  some  insulating  material,  as 
ebonite ;  and  at  each  end,  mounted  on  insulating  stems, 
are  brass  sliding  rods,  each  terminating  in  balls,  and 
passing  through  a  socket  hinged  on  the  top  of  its  stem. 
A  plaster  of  paris  receptacle,  to  hold  inflammable 
substances,  should  also  be  provided. 


88 


ELEMENTS  OF  STATIC  ELECTRICITY. 


The  substance  to  be  operated  on  is  placed  in  the 
receptacle  on  the  tablet,  the  inner  terminals  of  the 
sliding  rods  adjusted  on  opposite  sides  of  it,  and  the 
outer  terminal  of  one  rod  connected  with  the  outer 
coating  of  the  jar  or  battery ;  and  the  circuit  completed 
by  connecting  the  outer  terminal  of  the  other  rod  with  the 
knob  of  the  jar  or  battery,  by  the  discharger,  as  shown. 
The  wet  string,  or  other  imperfect  conductor,  when 

used,  can  be  intro- 
duced into  any  con- 
venient part  of  the 
circuit,  as  at  S. 

SPONTANEOUS  DIS- 
CHARGE.— A  sponta- 
neous discharge  is 
liable  to  occur  in 
attempting  to  charge 
a  jar  beyond  its  ca- 

Fig.  25— Universal  Discharger.  pacity  :     and,    if    the 

glass  is  thin  at  any  point,  it  may  be  fractured  in  this 
way ;  but  if  the  resistance  of  the  insulating  margin  is 
less  than  that  of  the  glass,  the  discharge  will  take  place 
over  that  surface,  without  injury  to  the  jar,  electricity 
always  following  the  path  of  least  resistance,  whether 
longer  or  shorter. 

DISRUPTIVE  DISCHARGE. — When  a  discharge  takes 
place  through  the  air  or  any  other  dielectric,  it  is 
termed  disruptive;  since  the  electricity  must  force  a 
passage  and  break  down  opposing  barriers.  Such  a 
discharge  is  always  accompanied  with  light,  heat,  and 
sound ;  as  expressed  by  the  terms  spark  and  snap,  flash 
and  report — effects  due  to  the  resistance  encountered, 
and  not  qualities  inherent  in  electricity. 


A  CCUMULA  TORS.  89 

SILENT  DISCHARGE. — But  when  the  discharge  takes 
place  through  a  good  conductor  of  sufficient  size,  it  is 
termed  silent;  since  light  and  sound  are  absent;  the 
resistance  encountered  being  only  sufficient  to  produce 
a  slight  amount  of  heat. 

The  discharge  through  a  point  is  also  termed  silent ; 
since  a  point,  as  already  shown,  offers  no  resistance ; 
and  hence  there  is  little  or  no  sound,  even  when  the 
discharge  passes  through  intervening  air.  A  battery 
discharge,  sufficient  to  destroy  life,  may  be  received 
with  impunity  through  the  point  of  a  cambric  needle, 
held  in  the  hand,  without  producing  any  unpleasant 
sensation. 

LICHTENBERG'S  FIGURES. — If,  on  a  plate  of  ebonite, 
or  of  glass  varnished  with  shellac,  figures  be  traced  with 
the  knob  of  a  positively  charged  Leyden  jar,  and  sulphur 
dusted  over  the  surface,  inclining  the  plate  and  tapping  it 
to  remove  the  surplus  ;  the  sulphur  will  adhere  to  the 
lines  traced,  spreading  out  in  a  beautiful  fringe,  as  shown 
in  Fig.  26,  which  is  from  a  photograph  of  a  figure  made 
in  this  way. 

A  similar  result  can  be  obtained  by  tracing  lines 
with  the  outside  of  this  jar,  or  with  the  knob  of  a 
negatively  charged  jar,  and  dusting  the  surface  with 
red  lead. 

Or  a  mixture  of  sulphur  and  red  lead  may  be  used,  and 
separate  figures  traced  ;  the  jar  being  charged  positively 
for  one  figure,  and  negatively  for  the  other.  The  sulphur, 
it  is  claimed,  adheres  to  the  positive,  and  the  lead  to  the 
negative  lines.  Any  non-conducting  surface  may  be 
used,  also  various  other  powdered  substances. 

It  should  be  noticed  that  the  loss  of  charge,  whether 
positive  or  negative,  from  the  inner  coating,  while  tracing 


90  ELEMENTS  OF  STATIC  ELECTRICITY. 

the  figures  with  the  knob,  is  balanced  by  an  equal  loss 
from  the  outer  coating  through  the  hand  in  which  the  jar 
is  held.  Hence,  when  the  tracing  is  made  with  the  outer 
coating,  the  knob  must  be  held  in  the  hand,  to  produce 
the  same  effect  on  the  inner  coating :  the  jar  being  first 
placed  on  an  insulator  to  prevent  a  discharge  and  conse- 
quent shock,  by  indirect  connection  through  the  earth. 


Fig.  26 — Lichtenberg's  Figures. 

An  inspection  of  the  figure  shows,  that  at  the  point 
where  it  begins  above,  the  fringe  lines  radiate  from  a  com- 
mon center ;  but  that,  as  the  curve  is  produced  from  right 
to  left  downward  and  from  left  to  right  upward,  they 
point  diagonally  in  the  direction  in  which  the  knob  of 
the  jar  moves.  The  explanation  is  as  follows : — The 


A  CCUMULA  TORS.  91 

surface  being  a  non-conductor,  the  electricity  has  to  force 
its  way  against  strong  resistance,  bursting  through  at  the 
points  where  resistance  is  least,  and  forming  the  fringe. 
The  strongest  effect  is  produced  where  the  knob  first 
approaches  the  surface  ;  as  the  jar  has  then  a  full  charge  : 
and  the  first  action  is  a  disruptive  discharge  through 
the  air,  producing  the  circular,  star-like  figure,  at  that 
point.  But  as  the  knob  moves  along  the  surface,  after 
contact,  new  lines  start  out  at  right  angles  to  the  line 
of  movement.  And  as  the  knob  leaves  a  point  where 
such  a  line  has  started,  it  exerts  an  inductive  action  on 
the  original  impulse,  which  tends  to  turn  this  line  for- 
ward; the  diagonal  direction  being  the  resultant  of 
these  two  forces  acting  at  right  angles  to  each  other. 
And  the  forked  branches  are  the  result  of  similar 
inductive  action  of  the  main  fringe  lines  on  the  branch 
lines.  • 

We  have,  in  this  experiment,  a  graphic  demonstration 
of  the  effect  of  an  insulating  surface  in  resisting  electric 
movement :  since  the  figures  show  the  exact  location  of 
the  electric  force ;  which,  we  see,  is  confined  chiefly  to 
the  tracings,  spreading  only  to  the  limited  extent 
represented  by  the  fringes. 

It  also  shows  that  the  effects  produced  in  different 
substances,  by  opposite  electric  influences,  are  depend- 
ent on  the  electric  condition  of  the  substances  them- 
selves: so  that  a  mixture  or  a  compound  may,  in  this 
way,  be  separated  into  its  elements.  The  sulphur  in 
this  experiment  becoming  negative,  as  claimed,  by 
friction,  is  attracted  to  the  positively  charged  lines, 
while  the  red  lead,  becoming  positive,  is  attracted  to 
those  negatively  charged.  This  principle  has  numer- 
ous useful  applications  in  the  arts. 


CHAPTER  VII. 
ELECTEIC  GENERATORS. 


THE  ELECTROPHORUS  AND  FRICTIONAL  MACHINE. 

THE  only  electric  generators  noticed  thus  far  are  the 
rods  of  glass,  ebonite,  and  sealing-wax ;  rubbed  with 
silk,  woolen,  or  fur:  but  it  is  evident,  that  for  such 
work  as  the  charging  of  Leyden  jars  and  batteries,  and 
similar  experiments,  we  require  generators  of  far  greater 
capacity.  But  it  was  thought  best  to  anticipate  their 
existence,  and  defer  their  introduction  till  there  had 
been  a  full  consideration  of  the  principles  on  which  the 

various  kinds  de- 
pend: so  that  they 
might  all  be  in- 
cluded in  one  com- 
prehensive view ; 
from  which  the 
merits  of  each,  and 
Fig.  27-Eiectrophorus.  the  principles  of 

its  construction  could  be  more  fully  ascertained. 

THE  ELECTROPHORUS. — This  instrument,  invented 
by  Volta,  is  one  of  the  simplest  forms  -of  a  static  gen- 
erator ;  but  it  is  of  great  utility  in  furnishing  an 
unfailing,  though  limited  supply  of  electricity,  for 
numerous  delicate  experiments. 

The  following  style,  designed  by  the  author,  and 
represented  by  Fig.  27,  makes  a  handsome,  convenient, 
and  very  efficient  instrument. 


ELECTRIC  GENERATORS.  93 

On  a  wooden  base  thirteen  inches  square,  constructed 
of  layers  glued  together  to  prevent  warping,  is  placed 
a  thin  sheet  of  brass  of  the  same  size  ;  over  which  is 
placed  a  sheet  of  ebonite  of  equal  size,  iV  of  an  inch 
thick ;  and  both  attached  to  the  base  by  screws  near 
the  corners. 

On  the  ebonite  is  placed  a  circular  plate  or  cover, 
made  of  No.  20  sheet  brass,  twelve  inches  in  diameter, 
perfectly  flat,  and  having  a  round  resistance  rim  joined 
to  the  upper  surface.  In  its  center  is  an  ebonite  handle, 
seven  inches  high ;  and  from  its  rim  projects  a  goose- 
neck, made  of  No.  8  brass  rod,  terminating  in  a  half- 
inch  brass  ball ;  near  which,  on  the  edge  of  the  base,  is 
a  brass  strip,  f  of  an  inch  wide,  connected  with  the 
lower  plate. 

The  base  may  be  made  of  metal,  if  preferred,  in  which 
case  the  lower  plate  and  strip  are  unnecessary,  the  base 
itself  taking  the  place  of  the  plate. 

In  this  instrument  we  have  two  conductors  separated 
by  a  dielectric ;  the  upper  one  insulated,  and  the  lower 
connected  with  the  earth. 

The  cover  being  removed,  the  dielectric  is  beaten 
briskly  with  a  piece  of  catskin,  or  other  fur,  by  which 
its  upper  surface  is  electrified ;  and  the  cover  is  then 
replaced. 

Suppose  the  charge  to  be  negative  ;  electricity  having 
been  removed  by  the  fur,  the  same  amount  is  attracted 
from  the  earth  to  the  under  surface  of  the  dielectric, 
and  to  the  upper  surface  of  the  brass  plate  in  connection 
with  it ;  which  thus  become  positive  by  induction.  The 
under  surface  of  the  cover  also  becomes  positive  and  its 
upper  surface  negative. 

Let  a  connection  now  be  made  between  the  lower 


94 


ELEMENTS  OF  STATIC  ELECTRICITY. 


plate  and  cover,  by  touching  the  strip  and  knob  with 
the  finger  and  thumb,  or  a  small  discharger ;  the  elec- 
tricity accumulated  on  the  lower  plate  will  pass  to  the 
cover,  producing  a  shock  if  passed  through  the  hand. 

The  cover  thus  becomes  positive ;  but  its  charge  is 
neutralized,  or  bound,  by  the  negative  of  the  dielectric. 
Let  it  be  lifted  off  by  the  insulating  handle  ;  its  charge 
being  no 'longer 'bound,  a  discharge,  producing  a  spark, 

an  inch  or  more  in 
length,  takes  place, 
when  the  knuckle 
or  any  conductor 
is  presented  to  the 
knob,  as  shown  in 
Fig.  28. 

The  removal  of 
the  cover  with  its 
positive  charge, 
having  left  the  up- 
per surface  of  the 
dielectric  negative, 
a  positive  charge 
is  again  attracted 
to  the  under  sur- 
face and  plate,  as  before ;  and  the  cover,  having  been 
discharged  and  replaced,  the  process  may  be  repeated 
with  the  same  results  an  indefinite  number  of  times, 
and  Leyden  jars  charged,  or  other  electric  work 
performed. 

Suppose  the  original  charge  to  be  positive,  the  same 
results  occur  in  reverse  order.  Electricity  having  been 
imparted  by  the  fur  to  the  upper  surface  of  the  dielec- 
tric, the  same  amount  is  repelled  from  the  under 


Fig.  28— Discharge  of  Electrophorus. 


ELECTRIC  GENERATORS.  95 

surface  and  plate,  making  them  negative.  The  under 
surface  of  the  cover  also  becomes  negative  and  its 
upper  surface  positive.  Connection  being  made  as 
before,  electricity  passes  from  the  cover  to  the  lower 
plate ;  leaving  the  cover  negative,  and  its  charge  bound 
by  the  positive  on  the  upper  surface  of  the  dielectric. 

The  cover  being  removed,  and  a  conductor  presented 
to  the  knob,  a  discharge  takes  place ;  electricity  now 
passing  from  the  conductor  to  the  cover,  instead  of  from 
the  cover  to  the  conductor  as  before. 

The  removal  of  the  cover,  with  its  negative  charge, 
having  left  the  upper  surface  of  the  dielectric  positive, 
electricity  is  again  repelled  from  the  under  surface  and 
plate  by  induction  :  and  the  cover  having  been  restored 
to  zero  and  replaced,  the  process  may  be  repeated  as 
before. 

We  see,  then,  that  when  the  charge  is  negative, 
electricity  is  attracted  from  the  earth  to  the  lower 
plate,  then  passes  to  the  cover,  and  then  from  the  cover 
to  the  presented  conductor;  but  when  the  charge  is 
positive,  electricity  is  repelled  to  the  earth  from  the 
lower  plate ;  then  an  equal  amount  passes  from  the 
cover  to  the  lower  plate,  and  the  same  amount  passes 
to  the  cover  from  the  presented  conductor. 

Hence,  when  the  instrument  receives  a  positive  charge, 
it  gives  a  negative  charge ;  and  when  it  receives  a  neg- 
ative charge,  it  gives  a  positive  charge. 

It  will  also  be  noticed  that  the  initial  charge  is  given 
by  friction,  but  all  subsequent  charges  are  obtained  by 
induction. 

If  the  cover  be  removed,  without  first  making  con- 
nection between  it  and  the  lower  plate,  no  charge  will 
be  found  on  it;  since  it  has  neither  gained  nor  lost 


96  ELEMENTS  OF  STATIC  ELECTRICITY. 

electricity  through  any  external  source ;  and  its  own 
electricity,  being  merely  changed  to  the  upper  or  lower 
surface,  by  the  positive  or  negative  of  the  dielectric,  is 
restored  to  zero  when  removed  from  that  influence. 

This  connection  may  be  made  automatically  fby  plac- 
ing a  short  brass  pin  in  a  hole  made  through  the  dielec- 
tric, its  upper  end  even  with  the  upper  surface,  so  that  it 
shall  touch  the  cover  and  also  the  lower  plate.  This 
makes  the  instrument  more  convenient  for  obtaining 
charges  in  rapid  succession :  but,  when  used  to  demon- 
strate the  principles  involved  in  its  construction,  as 
above,  the  pin  should  be  removed. 

The  top  of  the  handle  should  be  grasped,  when 
removing  the  cover,  to  prevent  a  partial  discharge 
through  the  hand. 

The  electrophorus  will  retain  its  charge  for  months ; 
and,  like  the  Leyden  jar  with  movable  coatings,  can  be 
taken  apart  and  put  together  again  without  perceptible 
loss  of  charge ;  but,  when  not  in  use,  the  charge  is 
gradually  dissipated,  so  that  only  a  residual  remains. 
Hence  it  should  be  charged  again  before  immediate 
use,  if  great  efficiency  is  desired.  This  property  of 
constancy  probably  suggests  the  name,  electrophorus, 
electricity-bearer,  from  qisaco  to  bear,  qfaxTQov  electricity. 

THE  FKLCTIONAL  MACHINE. — The  principle  of  this 
machine  is  the  same  as  that  of  the  rod  and  rubber.  It 
was  invented  by  Otto  Guericke,  and  consisted,  at  first, 
of  a  globe  of  sulphur,  revolved  on  an  axis  by  a  crank, 
the  hand  being  used  as  a  rubber.  Subsequently  a  globe 
of  glass  was  substituted  for  the  sulphur ;  but  as  insu- 
lation was  disregarded  in  both  styles,  only  feeble  results 
were  obtained,  and  the  machines  fell  into  disuse. 

Boze,  of  Wittemberg,  revived  and  improved  them, 


ELECTRIC  GENERATORS. 


97 


using  the  glass  globe,  and  a  band  wheel  and  belt  to 
increase  their  speed ;  and  collecting  the  electricity  on 
an  iron  tube,  suspended  by  silk  cords,  from  which  hung 
a  chain  in  contact  with  the  globe. 

Furt'  er  improvement  was  made  by  the  use  of  a 
leather  rubber  stuffed  with  hair :  and  subsequently  the 
globe  was  replaced  by  a  glass  cylinder,  on  one  side  of 


Fig.  29— Plate  Electrical  Machine. 

which  the  rubber  was  mounted  on  a  glass  pillar ;  and, 
on  the  other  side,  similarly  mounted,  was  a  brass  cylin- 
der, called  the  prime  conductor,  from  which  a  row  of 
points  projected  toward  the  glass.  An  oil  silk  flap 
enveloped  the  upper  part  of  the  glass  cylinder ;  and  a 
chain  was  used  to  connect  either  the  rubber  or  the 
prime  conductor  with  the  earth,  as  desired. 

The  plate  machine,  invented  about  1787,  was  con- 


98          ,:       ELEMENTS  OF  STATIC  ELECTRICITY. 

structed  'on  the  same  principles,  a  glass  plate  being 
substituted  for.  the  glass  cylinder,  and  has  now  come 
into  general  use.  Fig.  29  represents  one  of  the  prevail- 
ing styles. 

It  consists  of  a  disc  of  plate-glass  -A,  mounted  on  a 
wooden  base  with  wooden  or  glass  pillars,  and  revolved 
by  a  crank  with  an  insulated  handle.  A  pair  of  rub- 
bers B,  made  of  soft  leather  or  felt,  are  pressed  against 
the  glass  on  opposite  sides  by  a  pair  of  brass  springs  (7, 
the  pressure  being  adjusted  by  a  screw.  These  are 
mounted  on  a  glass  pillar,  and  connected  above  with  a 
brass  ball ;  and  a  brass  chain,  which  may  be  removed, 
connects  them  with  the  earth. 

Mounted  on  a  glass  pillar  is  the  prime  conductor  _Z), 
made  of  brass,  and  consisting  of  a  pair  of  balls,  from  the 
lower  one  of  which  projects  a  pair  of  combs,  which 
extend  on  opposite  sides  of  the  glass,  and  whose  teeth 
come  within  a  quarter  of  an  inch  of  it.  And,  from  the 
opposite  side  of  the  same  ball  extends  a  rod,  terminat- 
ing in  a  small  ball. 

A  silk  cover  envelops  the  lower  part  of  the  glass  plate, 
and  the  rubbers,  on  the  surfaces  in  contact  vvith  the  glass, 
are  coated  with  an  amalgam,  composed  of  five  parts 
zinc,  three  parts  tin,  and  nine  parts  mercury,  melted 
together,  pulverized,  and  made  into  a  paste  with  lard. 

The  machine  should  be  dry  and  warm  before  use,  as 
moisture  condenses  on  the  surface  of  the  glass  when  it 
is  colder  than  the  atmosphere,  and  suspends  insulation. 
For  this  reason  ebonite  pillars  have  an  advantage  over 
glass,  being  less  liable  to  condense  moisture. 

Ebonite  has  also  been  used  for  the  plate,  but  is  not 
so  reliable  as  glass ;  and  its  liability  to  warp  with  heat, 
when  in  thin  plates,  makes  it  very  objectionable. 


ELECTRIC  GENERATORS. 


ITS  MODE  OF  ACTION.—  The  plate  being  re^piy/ed'iil?  -y 
the  direction  of  the  arrow,  electricity  is  gener5tf«5^% 
the  friction  of  the  rubbers;  the  charged  surface  of  the 
glass  passing  directly  into  the  silk  cover,  which  prevents 
loss  of  charge  from  contact  with  the  air. 

O 

If  the  charge  on  the  glass  is  positive,  when  the 
charged  surface  comes  opposite  the  combs,  electricity 
passes  through  them  from  the  plate  to  the  prime  con- 
ductor, where  it  accumulates.  The  glass,  being  thus 
discharged,  passes  round  again  to  the  rubbers,  which, 
having  become  negative  from  parting  with  electricity 
to  the  glass,  have  received  electricity  from  the  earth 
through  the  chain. 

Each  portion  of  the  plate  is  thus  alternately  charged 
and  discharged,  as  it  passes  first  to  the  rubbers,  and 
then  to  the  combs;  the  lower  half  being  constantly 
positive,  and  the  upper  half  at  zero,  except  the  resid- 
ual ;  electricity  passing  to  the  rubbers  from  the  earth, 
and  being  carried  round  by  the  plate  to  the  prime 
conductor. 

If  the  charge  on  the  plate  is  negative,  the  transfer 
takes  place  in  reverse  order  ;  electricity  passing  from 
the  prime  conductor  to  the  plate,  from  the  plate  to  the 
rubbers,  and  from  the  rubbers  to  the  earth;  the  prime 
conductor  becoming  negative  and  the  rubbers  positive. 

If  the  prime  conductor  be  placed  in  connection  with 
the  earth,  by  having  the  chain  transferred  to  it,  the 
charge,  whether  positive  or  negative,  will  take  place  on 
the  ball  and  other  parts  connected  with  the  rubbers. 

If  the  prime  conductor  and  rubber  be  connected  by 
the  chain,  no  charge  can  occur  on  either;  since  elec- 
tricity constantly  passes  from  one  to  the  other  through 
the  chain,  as  it  is  generated. 


100  ELEMENTS  OF  STATIC  ELECTRICITY. 

If  the  chain  be  removed  entirely,  only  a  very  limited 
charge  can  occur,  derived  from  the  material  of  the 
machine  itself. 

The  limit  of  the  charge  is  reached  when  its  potential 
energy,  whether  positive  or  negative,  so  far  exceeds  the 
resistance  of  the  air,  that  the  loss  of  charge  by  convec- 
tion, as  explained  on  page  66,  shall  equal  the  energy 
generated.  When  the  atmosphere  is  damp,  or  its 
electric  potential  low,  this  limit  is  soon  reached;  but 
when  dry,  and  at  a  high  electric  potential,  a  much 
greater  charge  can  take  place. 

MACHINE  DESCRIBED  BY  NOAD. — The  largest  ma- 
chine of  this  kind  of  which  we  have  any  record  was 
made  some  years  ago  for  the  Panopticon  of  Science  in 
London.  According  to  Noad,  it  had  a  plate  ten  feet  in 
diameter,  three  pairs  of  rubbers,  each  three  feet  in 
length,  and  a  pear-shaped  prime  conductor,  six  feet 
in  length,  and  four  feet  in  diameter  at  its  widest 
part. 

It  was  operated  by  steam  power,  and  gave  sparks 
fifteen  to  eighteen  inches  in  length;  and  charged  to  its 
full  capacity,  in  less  than  a  minute,  a  Leyden  battery 
of  thirty-six  jars,  having  one  hundred  and  eight  square 
feet  of  coated  surface. 

MEASUREMENT  or  ENERGY. — The  amount  of  elec- 
tricity which  a  well-constructed  machine  can  generate 
is  in  proportion  to  the  surface  area  of  the  plate,  which 
may  be  increased  to  any  practicable  limit,  the  other 
parts  being  increased  in  like  proportion.  It  is  roughly 
estimated  by  the  number  of  sparks  of  a  given  length 
and  energy  which  can  be  obtained  in  a  given  time, 
when  an  uninsulated  conductor  is  brought  near  the 
prime  conductor;  or  by  the  length  of  time  required  to 


ELECTRIC  GENERATORS. 


101 


charge  a  Leyden  jar  or  battery  having  a  given  amount 
of  coated  surface. 

The  results  are  only  approximate,  especially  those 
by  the  first  method,  for  the  following  reasons.  Length 
of  spark  is  not  a  true  index  of  energy ;  since  a  short, 
thick  spark  may  have  greater  energy  than  a  long,  thin 
one:  and  our  estimate  of  the  comparative  energy  of 
each  from  its  appearance,  and  the  accompanying  snap, 
is  liable  to  be  very  inaccurate.  The  spark  accom- 


Fig.  30— Lane's  Unit  Jar. 

panying  the  discharge  of  a  Leyden  jar  or  battery  is 
generally  quite  short,  though  its  energy  often  greatly 
exceeds  that  of  any  single  spark  of  much  greater  length, 
given  by  the  machine  in  charging  it. 

The  humidity  of  the  air  and  its  electric  potential, 
being  liable  to  great  variation,  produce  a  corresponding 
variation  in  the  results  obtained  at  different  times. 

The  charge  and  discharge  of  a  Leyden  jar  of  a  given 
capacity,  in  a  given  time,  is  a  more  reliable  method. 


102 


ELEMENTS  OF  STATIC  ELECTRICITY. 


The  jar  should  be  made  self-discharging,  by  bringing 
the  knob  of  a  conductor,  connected  with  its  outer 
coating,  within  sparking  distance  of  the  knob  of  the 
jar. 

Lane's  unit  jar,  shown  in  Fig.  30,  is  constructed  on 
this  principle. 

A  bent  brass  rod  is  connected  by  a  band  to  the  outer 
coating;  its  upper  end  terminating  in  a  ball  through 
which  passes  a  horizontal  sliding  rod,  terminating  in  a 
ball  at  its  inner  extremity;  and  having  an  ebonite 
handle  at  its  outer  extremity,  by  which  the  ball  can 

be  adjusted  to  any  required 
distance  from  the  knob  of  the 
jar. 

To  estimate  the  comparative 
energy  of  different  machines, 
a  uniform  rotation  of  the  plates 
must  be  maintained  by  a  given 
number  of  revolutions  per  min- 
ute ;  and  the  number  of  dis- 
charges in  a  given  time  of  the 
unit  jar,  connected  with  the 
prime  conductor,  will  then  be  approximately  correct 
for  the  energy  of  each. 

THE  ELECTRIC  CHIME. — This  instrument  is  used  in 
connection  with  the  machine,  to  illustrate  electric 
attraction  and  repulsion. 

It  may  be  mounted  on  a  separate  stand,  or  hung  from 
the  projecting  rod  of  the  prime  conductor.  Fig.  31 
represents  a  common  style  used  in  this  way. 

It  consists  of  three  bells  suspended  from  a  brass  rod; 
the  two  outer  ones  by  brass  wires  or  chains,  and  the 
central  one  by  a  silk  cord ;  a  brass  chain  connecting  it 


Fig.  31— Electric  Chime. 


ELECTRIC  GENERATORS. 


103 


with  the  earth.  Between  the  central  and  outer  bells 
are  two  small  brass  balls,  suspended  by  silk  cords. 

When  the  machine  is  put  in  operation,  the  outer  bells 
receive  a  charge  from  the  prime  conductor;  this  acts 
inductively  on  the  insulated  balls,  which  are  at  zero, 
attracts,  and,  after  contact,  repels  them.  Being  now 
charged  the  same  as  the  outer  bells,  they  act  inductively 
on  the  central  bell,  repelling  or  attracting  electricity 
through  its  chain,  according  as  their  charge  is  positive 
or  negative  ;  and  pro- 
ducing on  it  a  charge 
of  the  opposite  kind, 
they  are  attracted  to  it 
and  discharged.  Be- 
ing now  at  zero,  they 
are  attracted  to  the 
outer  bells,  as  before  ; 
and  in  this  way  the 
three  bells  are  made 
to  ring. 

IMAGE  PLATES. — 
These    are    used    to 

show  the  effect  of  ill-  FiS-  32-Image  Plates. 

duction  between  two  conducting  surfaces,  as  repre- 
sented by  Fig.  32. 

From  the  projecting  rod  of  the  prime  conductor,  a 
brass  plate,  having  a  resistance  rim,  is  suspended  by  a 
wire  or  chain:  and  under  it.  on  an  insulating  stand,  is 
placed  another  similar  plate,  made  a  little  larger,  and 
joined  to  the  insulating  support  by  a  sliding  rod,  by 
which  the  distance  between  the  plates  may  be  adjusted, 
a  chain  connecting  it  with  the  earth. 

When  the  machine  is  put  in  operation,  the  upper 


104  ELEMENTS  OF  STATIC  ELECTRICITY. 

plate  will  have  the  same  charge  as  the  prime  conductor. 
If  the  charge  be  positive,  electricity  is  repelled  by  in- 
duction from  the  lower  plate  to  the  earth,  through  the 
chain ;  if  negative,  it  is  attracted  through  the  same 
medium ;  and,  in  either  case,  the  plates  are  oppositely 
charged  to  the  same  potential,  the  air  being  the  dielec- 
tric. 

When  the  space  is  properly  adjusted,  pith  balls  or 
images,  placed  on  the  lower  plate,  are  alternately  at- 
tracted and  repelled,  dancing  up  and  down  between 
the  plates  in  a  manner  which  is  often  quite  amusing. 

If  electric  connection 
with  the  earth  be  sev- 
ered by  removing  the 
chain,  this  effect  will 
cease :  which  proves 
that  the  opposite  poten- 
tials of  the  plates  was 
caused  by  the  transfer 
Fig.  33-The  Eteoric  Whirl.  of  electricity  to  or  from 

the  earth,  as  stated. 

THE  ELECTRIC  WHIRL. — This  little  instrument, 
shown  in  Fig.  33,  consists  of  a  set  of  pointed  brass  arms 
attached  to  a  common  center,  which  is  pivoted  on  the 
point  of  a  vertical  rod  connected  with  the  prime  con- 
ductor; the  arms  being  bent,  so  that  when  passing  a 
given  point  each  shall  turn  in  the  same  direction. 

When  the  machine  is  put  in  operation,  the  air  in 
front  of  each  point  becomes  electrified,  either  positively 
or  negatively,  by  the  passage  of  electricity  either  from 
or  to  the  point;  while  that  back  of  it  is  oppositely 
electrified  by  induction.  This  causes  repulsion  from 
the  air  in  front,  and  attraction  toward  that  at  the  back, 


ELECTRIC  GENERATORS.  105 

producing  rotation  of  the  instrument  in  the  opposite 
direction  to  that  in  which  the  points  turn. 

The  effect  of  a  stationary  point  in  producing  a  cur- 
rent of  air  is  shown  in  Fig.  34;  where  the  flame  of  a 
candle  is  represented  as  blown  from  a  point  attached 
to  the  prime  conductor. 

The  direction  of  the  air  current  will  be  the  same 
whether  the  charge  is  positive  or  negative :  since,  in 
either  case,  the  air  embraced  within  a  sphere  of  which 
the  point  is  the  center  will  have  the  same  potential  as 
the  prime  conductor;  while  that  outside  of  this  sphere 
will  assume  the  opposite  potential  by  induction.  Hence 
the  air  near  the 
point  becomes 
self-repellent, 
and  is  also  at- 
tracted by  the 
air  outside ;  that 
directly  in  front 

Of       the        point  Fig  34_Air  Current  from "a'print. 

being     repelled 

with  the  greatest  force,  produces  a  current  in  that  di- 
rection, while  the  air  on  either  side  is  attracted,  and,  in 
its  turn,  again  repelled. 

ARMSTRONG'S  HYDRO  -  ELECTRIC  MACHINE.  —  Fig. 
35  represents  a  machine  invented  by  Sir  William  Arm- 
strong, about  1840,  which  generates  electricity  by  the 
discharge  of  partially  condensed  steam. 

It  consists  of  a  boiler  and  furnace  mounted  on  glass 
pillars;  the  boiler  being  provided  with  steam  and  water 
gauges,  a  safety  valve,  and  a  condenser  inclosing  sev- 
eral small  pipes,  through  which  the  steam  escapes. 

These  pipes  are  surrounded  with  filaments  of  cotton, 


106 


ELEMENTS  OF  STATIC  ELECTRICITY. 


the  lower  ends  of  which  are  immersed  in  cold  water  at 
the  bottom  of  the  condenser :  and  the  water  being  thus 
raised  by  capillary  attraction,  cools  the  pipes,  producing 
partial  condensation  of  the  steam ;  thus  charging  it 
with  water  in  fine  drops,  by  the  friction  of  which 
against  the  pipes  electricity  is  generated;  the  steam 


Fig.  35 — Armstrong's  Hydro-Electric  Machine, 

being  discharged  against  a  row  of  points  connected 
with  the  prime  conductor. 

Each  pipe  is  furnished  with  a  wooden  tip :  and  the 
friction  is  increased  by  a  tongue  of  metal,  around  which 
the  steam  must  pass  before  entering  the  tip,  as  shown 
by  the  enlarged  section  at  letter  A. 

A  machine  of  this  kind,  constructed  for  the  Royal 


ELECTRIC  GENERATORS.  107 

Polytechnic  Institution  in  London,  had  a  boiler  seventy- 
eight  inches  long,  and  forty-two  inches  in  diameter,  with 
forty-six  steam  jets.  It  gave  sparks  twenty-two  inches 
in  length,  and  charged  to  its  full  capacity,  in  six  to  eight 
seconds,  a  Leyden  battery,  having  eighty  square  feet  of 
coated  surface. 

Another  one,  described  by  Noad,  had  one  hundred 
and  forty  steam  jets,  gave  sparks  of  the  same  length, 
with  thrge  or  four  times  the  rapidity;  and  charged,  to 
its  full  capacity,  a  Leyden  battery  having  1,188  square 
feet  of  coated  surface,  sixty  times  in  a  minute. 

But  though  capable  of  such  powerful  effects,  this 
machine  is  not  practical.  It  is  inconvenient  to  manage, 
requires  distilled  water,  careful  cleansing  of  the  boiler 
after  use,  and  great  steam  pressure.  Its  operation  is 
accompanied  with  a  deafening  noise,  and  the  escape  of 
a  great  volume  of  steam,  producing  dampness  and  other 
unpleasant  results,  when  used  in  a  room.  Hence  its 
chief  value  is  in  the  demonstration  of  the  important 
fact,  that  electricity  may  be  generated  in  this  way. 


CHAPTER  VIII, 
ELECTRIC   GENERATORS. 


THE  HOLTZ  AND  TOPLER  MACHINES. 

INFLUENCE  MACHINES. — Previous  to  1865,  frictional 
machines  were  the  principal  electro  -  static  generators 
in  use.  But  that  year  marked  an  era  in  electric  prog- 
ress by  the  invention  of  two  new  machines  of  remark- 
able energy,  by  the  German  electricians,  Holtz  and 
Topler;  to  which  the  name  influence  machines  was 
given,  from  their  being  constructed  with  two  or  more 
glass  plates,  arranged  to  generate  electricity  by  their 
mutual  inductive  influence. 

Both  machines  are  very  similar  in  construction ;  the 
principal  difference  being,  that  the  Holtz  requires  to  be 
incited  by  an  initial  charge  from  an  external  source, 
while  the  Topler  is  self-inciting. 

THE  HOLTZ  MACHINE. — This  machine,  of  which 
there  are  several  different  styles,  is  represented  by  Fig. 
36.  On  a  wooden  base  are  mounted  two  glass  plates  ; 
the  rear  plate  B  stationary,  and  supported  by  three 
ebonite  insulators,  two  below  and  one  above  ;  while  the 
front  plate  A  revolves  in  the  direction  of  the  arrow, 
on  a  steel  shaft,  which  passes  through  an  opening  in 
the  center  of  the  plate  B,  and  is  attached  to  the  post 
at  M.  A  is  mounted  on  an  ebonite  hub,  attached  to  a 
hollow  shaft  of  brass,  which  revolves  on  the  fixed  shaft, 
and  carries,  at  the  end  next  the  post,  a  small  pulley, 
from  which  a  belt  extends  to  the  driving  wheel,  which 


ELECTRIC  GENERATORS.  109 

is  revolved  by  a  crank  with  an  ebonite  handle.  The 
relative  sizes  of  the  wheel  and  pulley  are  such  as  to 
give  the  plate  four  to  six  revolutions  for  each  revolu- 
tion of  the  driving  wheel,  the  plates  of  small  ma- 
chines requiring  a  more  rapid  revolution  than  those  of 
larger  ones.  In  front  of  the  plate  A,  J  of  an  inch  from 
the  glass,  are  the  combs  T^and  H,  attached  to  a  brass 
core  at  the  center  of  the  ebonite  disc  M ;  and  the 
combs  IT  and  L,  insulated  by  their  attachment  to  ebon- 


Fig.  36-The  Holtz  Electric  Machine. 

ite  rods  projecting  from  the  disc  Jf,  and  connected  by 
brass  rods  with  the  Leyden  jars  C  and  D,  and  with  the 
sliding-rods  P  and  R.  These  sliding-rods  have  ebonite 
handles,  and  terminate  in  brass  balls  at  their  inner  ex- 
tremities. 

The  plates  are  of  sheet  glass,  about  \  of  an  inch  thick ; 
of  good  insulating  quality,  and  well  coated  with  shellac. 
The  stationary  plate  B  has  two  circular  openings  called 


110  ELEMENTS  OF  STATIC  ELECTRICITY. 

windows,  directly  opposite  the  combs  IT  and  L  ;  and,  on 
its  rear  surface,  are  cemented  two  paper  inductors  T 
and  X;  T  extending  from  H  to  L,  and  X  from  V  to  K ; 
and  each  armed  with  a  row  of  points,  projecting  into 
each  window. 

Machines  of  this  kind  are  often  constructed  with 
more  than  two  plates ;  sometimes  with  a  large  number. 
The  plates  are  also  sometimes  placed  in  a  horizontal 
position.  Ebonite  plates  are  also  used ;  but  are  objec- 
tionable, for  reasons  already  given. 

THE  TOPLER  MACHINE. — The  Topler  machine  has 
the  same  general  construction  as  the  Holtz ;  but,  on 
the  front  surface  of  the  revolving  plate,  are  cemented 
a  number  of  small  metal  discs,  called  carriers;  usually 
made  of  tin-foil  with  raised  brass  centers,  which,  as  the 
plate  revolves,  are  brought  into  contact  with  four  wire 
brushes;  two  attached  to  the  stationary  plate,  and  two 
to  the  uninsulated  combs.  In  this  way  the  machine  is 
made  self-inciting,  as  already  mentioned. 

The  windows,  and  the  rows  of  points  projecting  into 
them,  used  in  the  Holtz  stationary  plate,  are  omitted 
from  the  stationary  plate  of  the  Topler:  and  the  paper 
inductors  are  made  longer,  and  have  small  tin-foil  in- 
ductors under  them,  connected,  by  tin-foil  strips,  with 
each  other  and  also  with  the  two  brushes  attached  to 
this  plate. 

Fig.  37  represents  a  Topler  machine  constructed  by 
the  author,  and  patented  April  10,  1883,  and  December 
8, 1885.  The  principal  points  covered  by  the  patents 
are  as  follows  :  — 

1.  The  outside  coatings  of  the  Leyden  jars  C  and 
D  are  of  sheet  brass,  f  nickel  plated ;  and  are  screwed 
firmly  to  the  base  ;  forming  cups  into  which  the  jars 


ELECTRIC  GENERATORS. 


Ill 


fit  closely,  and  are  thus  held  in  a  fixed  position  ;  afford- 
ing a  firm  support  to  the  parts  connected  with  them, 
and  preventing  liability  to  accident  or  injury  to  the  jars 
or  plates. 

2.  The  induced  current  from  these  outside   coatings 
is  conveyed  down  by  the  brass   screws  which  attach 


Fig.  37— Atkinson's  Topler  Electric  Machine. 

them,  and  along  copper  wires  underneath,  to  the  termi- 
nals of  the  switch  S ;  through  which,  when  closed,  it 
passes  from  one  jar  to  the  other ;  but  when  open,  as  in 
the  cut,  it  passes  by  the  brass  sockets,  seen  on  the  edge, 
which  are  also  connected  with  the  terminals,  out 
through  the  conducting  cords,  and  a  person,  or  other 


112  ELEMENTS  OF  STATIC  ELECTRICITY. 

object,  connected  with  their  outer  extremities.  As  this 
induced  current  flows  simultaneously  with  the  direct 
current  from  the  inside  coatings,  the  switch  and  sliding- 
rods  place  it  completely  under  control  of  the  operator. 

3.  The  brush  holders,  E  and  F,  are  attached  to  the 
plate  B,  through  holes  near  its  edge  ;   thus  giving  a  di- 
rect passage  to  the  electricity  from  the  carriers  on  the 
plate  A,  where  it  is  generated,  through  the  glass,  to  the 
tin-foil  inductors,  represented  by  the  dark  shade,  and 
the  paper  inductors  T  and  X,  represented  by  the  light 
shade.     By  passing  the  electric  charge  through  the  glass, 
inside  its  edge,  an  insulating  margin  is  interposed  be- 
tween the  conductors  and  the  edge,  thus   preventing 
loss  from  leakage,  which  is  unavoidable  when  the  brush 
holders  are  attached  by  clamps  or  ears  on  the  edge. 

4.  The  carriers  on  the  plate  A  are  of    sheet  brass, 
with  raised  centers,  and  are  nickel  plated,  making  them 
both  durable  and  ornamental.     The  hard  nickel  surface 
is  not  affected  by  the  action  of  the  brushes,  or  the  elec- 
tricity, while  tin-foil  soon    becomes  defaced:  and  the 
carrier,  being  practically  one  piece,  and  its  entire  sur- 
face cemented  to  the   glass,  its  raised  center  cannot  be- 
come detached,  as  may  happen  when  the  center  is  put 
on  separately  over  a  tin-foil  base. 

5.  The  combs  V  and  K,  also  H  and  L,  radiate  at  an 
angle  of  45  degrees  to  each  other,  from  the  central  disc 
M,  to  which  they  are  attached ;  so  that  any  possibility 
of  error  in  regard  to  their  position,  or  of  displacement,  is 
practically  impossible. 

The  following  improvements  may  also  be  noticed:  — 
The  base  is  made  of  two-inch  strips,  glued  together 

lengthways,  and  heavy  cleats  screwed  on  underneath ; 

giving   all  the  advantages  of  iron  as  to  freedom  from 


ELECTRIC  GENERATORS. 


113 


warping,  with  the  insulation  and  elegant  finish  of  the 
wood.  The  driving  wheel  is  of  ebonite  and  the  iron 
casting,  on  which  it  is  mounted,  slides  in  grooves  on  an 
iron  plate,  and  is  moved  by  the  adjusting  screw  0,  to 
regulate  the  tension  of  the  belt. 


Fig.  38— Atkinson's  Four-Plate  Topler  Machine— Front  View. 

The  ebonite  insulators,  which  support  the  plate  J9,have 
soft  rubber  packing,  to  ease  the  pressure  on  the  glass. 

The  conducting  rods  of  the  Leyden  jars  pass  through 
ebonite  caps  with  cork  attached  underneath,  which 
gives  them  a  fixed  vertical  position,  and  affords  firm 
support  to  the  sliding-rods  and  the  combs  connected 
with  them  above. 


114 


ELEMENTS  OF  STATIC  ELECTRICITY. 


THE  FOUR-PLATE  TOPLER  MACHINE. — This  machine 
has  the  same  construction  in  front  as  the  two-plate  ma- 
chine as  shown  by  Fig.  38,  but  a  special  construction 
for  the  two  rear  plates  which  will  be  understood  by 
reference  to  Figs.  39  and  40. 

The  end  view,  Fig.  39,  shows  two  pairs  of  plates,  the 
position  of  the  rear  pair  being  reversed,  which  brings 
the  stationary  plates  into  the  center,  back  to  back,  be- 
tween the  revolving  plates ; 
so  that  the  inductors  are  on 
the  inner  surfaces  of  the 
stationary  plates,  and  the 
carriers  on  the  outer  sur- 
faces of  the  revolving  plates, 
which  being  mounted  on 
the  same  shaft,  with  a  col- 
lar between  them,  revolve 
in  unison. 

The  combs  L  and  K,  and 
F'and  H,  have  curved  rods 
L   which      pass      round      the 
plates    and    support    dupli- 
cate   combs   in  the  rear  as 

Figi  30_Atkinson's  Four-Plate        shown    ill    the     Cuts.        The 
Topler  Machine-End  View.          brusnes   are  algo    duplicated 

as  shown :  so  that  with  the  exception  of  the  Leyden 
jars  and  switch,  and  parts  connected  with  them,  this 
is  practically  a  double  machine. 

In  like  manner  an  eight-plate  machine  may  be  made 
by  doubling  these  parts  of  the  four-plate. 

When  the  large  Topler  or  Holtz  machines  are 
wanted  for  constant  use,  the  motive  power  is  usually 
supplied  by  a  steam  or  gas  engine,  or  a  water  motor. 


ELECTRIC  GENERATORS. 


115 


In  which  case  the  driving  wheel  is  not  used ;  the  belt 
passing  directly  from  the  small  pulley  connected  with 
the  plates,  to  a  pulley  attached  to  the  engine  or  motor. 

MODE  OF  ACTION  OF  THE  TOPLER. — To  compre- 
hend the  action  of  any  electric  generator,  the  following 
essential  principles  in  their  construction  should  be  kept 
distinctly  in  view. 

To  generate  electricity,  is  to  create  a  difference  of 
electric  potential ;  the  efficiency  of  all  generators, 


Fig.  40— Atkinson's  Four-Plate  Topler  Machine— Rear  View. 

whether  batteries,  dynamos,  or  glass  plate  machines, 
depending  on  the  difference  of  potential  which  each  is 
able  to  create  and  maintain  within  the  apparatus  itself. 
And  the  work  to  be  done  by  such  an  apparatus  is  the 
restoration  of  equilibrium,  through  an  exterior  circuit ; 
and  may  consist  in  producing  heat  or  light,  chemical, 
mechanical,  or  physiological  action. 

Let  us  consider  how  these  principles  apply  to  this 
machine. 


116  ELEMENTS  OF  STATIC  ELECTRICITY. 

Fig.  37,  page  111,  represents  the  machine  with  the 
sliding  electrodes  P  and  R  separated.  Suppose  the 
switch  AS'  to  be  closed  and  the  machine  put  in  operation. 
It  will  be  seen  that  as  the  plate  A  revolves,  the  raised 
centers  of  the  six  carriers  are  brought  into  contact  with 
the  wire  brushes  attached  to  the  holders  E  and  F;  each 
opposite  pair  touching  opposite  brushes,  successively, 
at  the  same  instant.  The  friction  generates  electricity, 
which  diffuses  itself  over  the  carriers  on  A,  and  the  in- 
ductors on  J5,  with  which  they  are,  at  the  instant  of 
contact,  in  electric  connection.  The  potential  of  car- 
rier and  inductor,  during  contact,  will  be  the  same ;  at 
the  next  instant  the  carrier  passes  on,  and  is  insulated 
from  the  inductor,  and  carrier  and  inductor  now  act  in- 
ductively on  each  other,  and  multiply  the  initial  charge 
given  by  the  friction  of  contact.  As  it  accumulates,  it 
spreads  over  the  paper  inductors  ;  these  act  on  the 
opposite  surfaces  of  the  glass,  till  both  surfaces  of  both 
plates  become  charged ;  the  initial  charge  being  still 
continued  by  the  constant  friction  of  the  carriers  and 
brushes. 

But,  since  both  sides  of  the  machine  are  of  similar 
construction,  and  since  the  mode  of  action  on  both 
sides  is  apparently  the  same,  the  question  arises,  how 
any  difference  of  potential,  or  electric  charge  can  be 
accounted  for. 

And  first,  it  will  be  noticed,  that  the  position  of  the 
plates  being  vertical,  their  lower  halves  are  nearer  to 
the  earth,  by  their  semi  diameter,  than  the  upper  halves, 
and  consequently,  more  under  the  influence  of  its  in- 
ductive action,  by  the  square  of  that  distance.  The 
lower  halves  are  also  in  close  proximity  to  the  Leyden 
jars,  the  driving  wheel,  and  the  belt,  and  subject  to  their 


ELECTRIC  GENERATORS.  117 

inductive  influence;  and  the  plate  B  is  supported  on  two 
insulators,  while  the  upper  half  has  but  one,  and  hence 
has  the  advantage  of  the  better  insulation  of  the  air. 

To  this  lower  half  of  B,  and  subject  to  these  influ- 
ences, is  attached  the  brush  holder  F,  while  E  is 
attached  to  the  upper  half,  and  remote  from  them. 
Hence,  the  carriers  brushed  by  E,  and  descending  to- 
wards L,  must  acquire  a  higher  potential  than  those 
brushed  by  F,  and  ascending  towards  K. 

An  accumulation  of  electricity  must  also  occur  at  the 
lower  ends  of  the  inductors  ^and  X,  from  the  induct- 
ive influence  of  the  earth ;  and  as  the  brush  holder  F 
is  placed  at  the  lower  end  of  X,  it  furnishes  an  outlet 
to  a  portion  of  this  charge,  as  seen  at  night  by  the 
brushes  of  light  from  this  holder  to  the  outside  of  the 
jar  (7,  and  other  parts  in  close  proximity. 

The  lower  end  of  T,  on  the  contrary,  is  well  insu- 
lated ;  hence  the  potential  of  T,  from  the  heavier  charge 
at  its  upper  end,  and  the  better  insulation  at  its  lower 
end,  must  be  much  higher  than  that  of  X,  where  the 
influences  are  just  the  reverse. 

This  accumulation,  or  high  positive  potential  at  the 
lower  end  of  T,  produces  a  high  negative  potential  at 
that  point  on  the  plate  A,  and  its  carriers,  as  it  revolves  ; 
as  shown  by  the  brush  of  light,  seen  in  the  dark,  from 
the  uninsulated  comb  F,  marking  the  flow  of  electricity 
to  the  upper  part  of  the  plate,  as  it  passes  under  that 
comb;  the  outflow  of  the  current  received  through  the 
comb  H.  This  brush  of  light  extends  downward,  as 
the  charge  increases,  almost  to  the  comb  K:  and  a  sim- 
ilar brush  extends  downward  from  K,  marking  the 
outflow  of  electricity  from  the  interior  of  the  jar  (7,  as 
explained  hereafter:  while  the  points  of  the  combs,  L 


118  ELEMENTS  OF  STATIC  ELECTRICITY. 

and  H,  where  the  charge  is  received,  show  only  a  glow 
of  light. 

These  brushes  of  light  always  turn  in  the  opjtosite 
direction  to  that  in  which  the  plate  A  revolves ;  differ- 
ence of  potential  between  the  comb  and  that  portion  of 
the  plate  approaching  it  producing  attraction ;  while 
equality  of  potential  between  the  comb  and  that  portion 
of  the  plate  receding  from  it  produces  repulsion. (seep. 224.) 

Following  any  opposite  pair  of  carriers,  as  TFand  Z,  we 
find  that  as  Z  passes  under  the  wire  brush  _F,  W  passes 
under  E ;  and  as  Z  moves  on  to  the  insulated  comb  7T, 
TFat  the  same  instant  arrives  at  L  ;  but  Tf,  as  already 
shown,  has  a  higher  potential  than  Z,  and,  at  this  point, 
a  peculiar  adjustment  takes  place.  W  gives  up  its 
charge  through  the  comb  L,  to  the  inside  of  the  Ley- 
den  jar  D.  This  creates  a  positive  charge  on  the  inside 
of  .Z>,  which  induces  a  negative  charge  on  its  outside. 
The  electricity  thus  repelled,  passes  to  the  outside  of 
(7,  making  it  positive,  and  inducing  negative  on  its 
inside ;  and  this  repelled  electricity  flows  through  the 
comb  K  to  the  plate  A,  as  already  shown.  W  then 
moves  down  to  the  uninsulated  comb  ZT,  while  Z  moves 
up  to  V.  Each  now  passes  under  the  wire  brush  at- 
tached to  its  respective  comb,  and  the  combs  being 
attached  to  the  brass  core  at  the  center  of  M,  the  carriers 
are  put  in  electric  connection  with  each  other,  and  their 
potential  equalized  by  the  flow  of  the  residual  charge 
from  If  to  F",  as  already  described;  so  that  each  arrives 
at  the  original  position  of  the  other  at  the  same  poten- 
tial, ready  to  repeat  the  same  process. 

It  should  be  noticed,  that  the  residual  is  slightly  in- 
creased by  induction  from  Tand  X,  as  the  carriers  move 
from  the  combs  L  and  .Zf  to  the  combs  JTand  F". 


ELECTRIC  GENERATORS.  119 

The  surfaces  of  the  plates,  on  which  the  carriers  and 
inductors  are  mounted,  assume  the  same  potential  as 
the  carriers  and  inductors  attached  to  them,  while  their 
opposite  surfaces  have  the  reverse.  Opposite  parts  of 
the  same  surface  are  also  in  opposite  electric  states :  the 
section  L  M  H,  for  instance,  having  a  potential  oppo- 
site to  that  of  V  M  K;  change  of  potential  on  these 
surfaces  following  that  of  the  carriers  and  inductors, 
already  described. 

It  will  be  noticed  that  the  office  of  the  brushes,  E  and 
F,  is  the  reverse  of  that  of  H  and  F".  E  and  F  generate 
by  friction,  while  H  and  V  discharge  by  contact.  And, 
while  the  combs,  K  and  L,  aid  in  creating  a  difference 
of  potential,  the  combs,  H  and  F",  aid  in  restoring  equi- 
librium. 

When  the  difference  of  potential  between  the  inner 
coatings  of  the  jars  becomes  sufficient  to  overcome  the 
resistance  of  the  air,  a  discharge  from  the  inner  coating 
of  D  to  that  of  C  takes  place  between  the  terminals  of 
the  sliding  electrodes  R  and  P ;  and,  at  the  same 
instant,  a  discharge  from  the  outer  coatings  takes  place 
through  the  switch  and  connections,  from  0  to  D,  to 
restore  equilibrium  between  them,  and  thus  complete 
the  circuit. 

A  spark  and  snap,  from  the  resistance  of  the  air, 
accompanies  the  discharge  between  the  inner  coatings ; 
and  the  same  will  occur  between  the  outer  coatings  if 
the  switch  is  open ;  but,  if  closed,  the  discharge  takes 
place  silently.  The  plates  and  other  parts  being,  at  the 
same  instant,  relieved  of  strain,  there  is  a  restoration  of 
equilibrium  in  the  whole  machine. 

The  above  explanation  applies  to  the  machine  when 
it  is  put  in  operation  from  a  state  of  absolute  rest ;  but 


120  ELEMENTS  OF  STATIC  ELECTRICITY. 

when  it  is  in  a  high  state  of  activity,  there  frequently 
occurs  a  reversal  of  potential  after  a  discharge,  as  shown 
by  the  reversal  of  the  brushes  of  light  from  the  combs. 
To  account  for  this  it  must  be  considered,  that  the 
residual  which  remains  after  the  primary  discharge  may, 
from  unequal  resistance,  be  greater  on  one  side  than  on 
the  other;  and  after  being  relieved  from  strain  by  the 
primary  discharge,  it  will  operate  to  give  a  slight  pre- 
ponderance of  potential  to  that  side,  which  is  rapidly 
multiplied  by  induction,  as  the  rotation  of  the  plate 
continues. 

A  reversal  can  also  be  produced  by  a  temporary 
reversal  of  rotation,  as  explained  on  page  140 ;  or  by 
touching  the  inductors,  or  parts  connected  with  them, 
while  in  action,  which  would  reduce  the  potential  at  that 
point.  Special  conditions  may  also  exist  in  certain 
machines,  which  will  reverse  the  ordinary  mode  of 
action ;  as,  for  instance,  a  difference  of  thickness  on 
opposite  parts  of  a  glass  plate  ;  or  in  opposite  jars. 

It  should  be  noticed  that  the  electric  charge  is 
instantly  diffused  over  the  metal  carriers  and  inductors, 
more  slowly  over  the  paper  inductors,  and  still  more 
slowly  over  the  shellacked  surfaces  of  the  glass  plates. 
So  that  when  the  machine  is  put  in  action,  after  a  con- 
siderable interval  of  rest,  three  or  four  seconds  elapse 
before  it  becomes  fully  charged,  and  a  crackling  sound 
is  heard  from  the  electricity  forcing  itself  over  the 
resisting  surfaces  of  the  paper  and  glass. 

The  condition  of  the  air,  as  to  its  insulation,  influ- 
ences the  whole  operation  of  this  machine.  An  air 
space  insulates  the  plates,  and  also  the  jars,  with  their 
rods  and  balls,  from  each  other ;  and  as  a  damp  atmos- 
phere lessens  this  insulation,  it  will  decrease  the  energy 


ELECTRIC  GENERATORS.  121 

of  the  machine  in  like  proportion.  A  film  of  moisture, 
settling  on  the  plates,  will  often  so  reduce  the  insula- 
tion, that  the  slight  initial  charge  by  the  action  of  the 
brushes  is  conducted  over  the  damp  surface  as  fast  as 
it  is  generated ;  so  that  no  difference  of  potential,  and 
consequently  no  permanent  charge,  can  occur.  And  as 
the  machine  is  much  more  sensitive  to  such  influences 
than  the  operator,  the  latter  is  often  puzzled  to  know 
why  it  will  not  generate.  The  simple  and  effectual 
remedy,  in  all  such  cases,  is  to  dry  it.  This  may  be 
done  by  a  fire,  a  kerosene  lamp,  a  hot  iron,  or  by 
the  sun's  heat,  though  artificial  heat  is  generally  more 
effectual. 

Warm  days,  before  or  after  rain,  when  the  atmos- 
phere is  loaded  with  moisture,  are  the  most  unfavorable. 
At  such  times  the  plates  should  not  only  be  dried,  but 
warmed,  as  moisture  will  continue  to  be  deposited  so 
long  as  they  are  colder  than  the  air. 

The  electric  conditions  in  upper  rooms,  other  things 
being  equal,  are  more  favorable  to  the  operation  of  the 
machine  than  in  those  on  the  ground  floor. 

MULTIPLICATION  or  THE  CHARGE. — The  multipli- 
cation of  the  initial  charge  proceeds  with  great  rapidity. 
During  the  first  revolution  of  the  plate  A,  each  tin-foil 
inductor  receives  six  direct  charges  from  the  contact  of 
its  connecting  brush  with  each  of  the  six  carriers:  and 
also  six  inductive  charges  of  equal  amount,  as  each 
charged  carrier  passes  it.  So  that  at  the  end  of  the 
first  revolution,  it  has  accumulated  twelve  charges  ;  and, 
during  that  revolution,  it  has  reacted  inductively  on  each 
passing  carrier  with  this  constantly  increasing  energy, 
increasing  the  energy  of  the  carrier  in  like  proportion. 

At  the  beginning  of  the  second  revolution,  it  has 


122  ELEMENTS  OF  STATIC  ELECTRICITY. 

twelve  times  the  inductive  energy  which  it  had  at  the 
beginning  of  the  first;  and  this  energy  continues  to 
increase,  and  react  on  the  carriers,  at  the  same  rate  as 
before.  And  as  the  plate  makes  about  five  revolutions 
per  second,  the  rate  of  increase  on  any  tin-foil  inductor 
is  about  sixty  increments  per  second. 

But  as  the  charge  spreads  from  the  tin-foil  inductors 
over  the  paper  inductors  and  adjacent  parts  of  the  sta- 
tionary plate;  and  from  the  carriers  over  adjacent  parts 
of  the  revolving  plate,  each  point  on  each  plate,  within 
the  charged  areas,  becomes  a  center  of  direct  and  induct- 
ive action  in  the  same  manner  as  the  metal  inductors  and 
carriers.  So  that  even  an  infinitesimal  charge  is  increased 
in  a  few  seconds  to  the  full  capacity  of  the  machine. 

HOLTZ  AND  TOPLER    MACHINES  COMPARED. — Since 

the  chief  difference  between  the  Holtz  and  Topler  con- 
sists in  the  latter  being  self-inciting,  the  mode  of  action 
is  essentially  the  same  in  each. 

The  Holtz  may  receive  its  initial  charge  from  a  fric- 
tional  machine,  an  electrophorus,  or  any  similar,  exter- 
nal source :  but  the  usual  method  of  charging  is  by 
means  of  a  piece  of  ebonite,  electrified  by  the  fur  of  a 
cat-skin. 

The  electrified  ebonite  is  held  in  contact  with  one  of 
the  paper  inductors  on  the  stationary  plate,  which  is 
thus  charged ;  a  portion  of  the  charge  being  commu- 
nicated to  the  revolving  plate  through  the  points  which 
project  into  the  windows;  and  this  plate  is  made  to 
rotate  rapidly,  so  that  the  charge  is  soon  multiplied  to 
the  full  capacity  of  the  machine,  if  the  atmospheric  con- 
ditions are  favorable;  and  the  ebonite  is  then  removed. 

It  will  thus  be  seen  that  the  initial  charge  in  both 
machines  is  produced  by  friction  and  multiplied  by 


ELECTRIC  GENERATORS.  123 

induction.  In  the  Holtz  it  is  derived  from  an  external 
source,  begins  011  the  stationary  plate,  and  is  then  com- 
municated to  the  revolving  plate.  In  the  Topler  it  is 
produced  by  the  machine  itself,  begins  on  the  revolving 
plate  and  is  then  communicated  to  the  stationary  plate. 
In  the  Holtz  it  occurs  on  one  side  only.  In  the  Topler  it 
is  simultaneous  on  both  sides.  In  the  Holtz  it  ceases  when 
the  plates  are  charged.  In  the  Topler  it  is  continuous. 

The  absence  of  the  brushes,  carriers,  and  metal  in- 
ductors from  the  Holtz  increases  the  internal  resist- 
ance, making  it  more  difficult  to  charge,  but  giving 
better  insulation,  and  consequently  greater  energy  than 
a  Topler  of  the  same  size. 

But  the  action  of  a  Holtz  is  much  more  liable  to 
interruption  from  dampness,  and  a  low  electric  poten- 
tial in  the  atmosphere :  since  it  receives  only  a  small 
initial  charge,  which  is  soon  discontinued ;  while  that 
of  the  Topler  is  constant,  from  the  continuous  action 
of  the  carriers  and  brushes.  So  that  a  well  constructed 
Topler,  with  ordinary  care,  is  reliable  in  any  state  of 
the  atmosphere,  while  a  Holtz  is  very  unreliable. 

COMPARISON  BY  DR.  HOLTZ. — In  reply  to  an 
inquiry  as  to  whether  the  Topler  machine  was  an 
original,  independent  invention,  or  only  a  modification 
of  the  Holtz,  the  author  received  a  letter  from  Dr. 
Holtz,  written  from  Greifswald,  Germany,  March  20, 
1883;  in  which  he  says,  that  his  machine,  as  first  de- 
scribed in  PoggendorfTs  Annalen,  in  1865  (volume 
125,  page  469,  and  volume  126,  page  157),  had  "two 
discs  rotating  in  opposite  directions,  without  stationary 
discs  "  ;  and  that  "  The  Topler  machine,  invented  at  the 
same  time,  was  a  combination  of  two  pairs  of  discs"; 
two  movable  and  two  stationary. 


124  ELEMENTS  OF  STATIC  ELECTRICITY. 

He  then  says  : — 

"  Topler  has  recently  rejected  his  system  and  adopted  mine, 
because  it  is  simpler,  and,  at  the  same  time,  more  effective.  The 
application  of  the  pointed  combs  and  the  non-covered  movable 
discs  is  also  my  invention,  since  the  Topler  machine  had  only  the 
tin-foil  coverings  and  sliding  springs.  (Schleifende  Federn.) 

11 1  had  been  accustomed  to  the  same,  indeed,  already ;  although 
not  with  independent  acting,  influence  machines,  but  rejected 
them  on  account  of  the  smaller  spark-length. 

"  Topler  has  also  lately  adopted  my  principle  of  the  pointed 
combs,  and  the  non-covered  discs  ;  but  so  far  modified,  that  besides 
the  pointed  combs  and  non-covered  discs,  he  yet  allows  to  act,  at 
the  same  time,  small  pieces  of  tin-foil  (or  pieces  of  metal),  and  the 
sliding  springs.  This  has  the  advantage  that  the  machine  excites 
itself,  and  is  less  sensitive  to  moisture ;  but  also  the  great  disad- 
vantage, that  the  sparks  become  shorter,  and  a  constant  reversal 
of  current  follows.  Besides,  a  certain  mechanic,  Voss,  also  claims 
this  machine,  so  modified,  as  his  merit ;  but  unquestionably  Topler 
was  the  first  who  showed  that  influence  machines,  with  metallic 
covering  and  sliding  springs,  excite  themselves. 

"  The  entire  form  of  the  machine,  its  symmetrical  construction, 
the  one-sided  support  of  the  axis,  the  application  of  a  sheath 
running  upon  a  pin  fastened  on  one  side,  the  application  of  the 
so-called  rotary  diametrical  (double)  pointed  combs,  the  applica- 
tion of  the  so-called  condensers  (small  Leyden  jars)  for  increase  of 
spark-length,  is  all  mine,  as  published  in  the  year  1869,  by  Professor 
Poggendorff  (PoggendorfFs  Annalen,  vol.  136,  page  171). 

"  Yours  truly,  DR.  W.  IIOLTZ." 

The  "  sliding  springs  "  mentioned  above,  doubtless 
refers  to  a  style  of  construction  in  which  the  springs 
glide  continuously  over  the  surface  of  the  glass  ;  essen- 
tially different,  and  differing  in  its  effect,  from  that  of 
the  brushes,  which  touch  only  the  raised  centers  of  the 
carriers,  and  are  wholly  insulated  from  the  glass; 
giving  alternate  contact  and  insulation,  making  induc- 
tion much  more  effective.  The  latter  construction  is 
attributed  to  Voss. 


CHAPTER 
EXPERIMENTS  WITH  THE  TOPLER  MACHINE. 

IN  experiments  with  the  frictional  machine,  such  as 
the  charging  of  Leyden  jars,  and  the  ringing  of  bells, 
as  already  described,  induction  is  produced  by  connect- 
ing one  part  of  the  apparatus  with  the  earth,  and 
another  part  with  the  prime  conductor.  But  in  the 
Holtz  and  Topler,  the  charge  is  accumulated  in  the 
Leyden  jars  instead  of  on  a  prime  conductor ;  and  any 
change  of  potential  in  one  jar  must  be  compensated  by 
a  corresponding  inductive  change  in  the  opposite  jar. 
Hence  to  obtain  the  full  inductive  effect,  connection 
must  be  made  with  the  opposite  jars. 

For  convenience  in  making  this  connection,  holes  are 
drilled  in  the  knobs  surmounting  the  jars,  and  the 
charge  is  conveyed  by  insulated  conducting  cords,  hav- 
ing brass  tips  which  fit  these  holes. 

Thus,  by  connecting  the  inner  and  outer  coatings  of 
a  Leyden  jar  or  battery  with  the  opposite  jars  in  this 
way,  a  full  charge  can  be  given  very  rapidly. 

In  a  similar  manner,  image  plates,  bell  chimes,  and 
other  apparatus,  mounted  on  separate  stands,  can  be 
connected  and  used. 

ELECTRIC  CHTME  FOR  TOPLER  MACHINE. — Fig.  41 
represents  a  chime  designed  by  the  author,  which  is 
mounted  on  the  machine  itself.  It  consists  of  two 
brass  arms  A  and  J5,  insulated  by  an  ebonite  connector 


126 


ELEMENTS  OF  STATIC  ELECTRICITY. 


C ;  the  tips  of  the  arms  being  fitted  to  the  holes  in  the 
knobs  of  the  jars. 

A  bell  is  suspended  from  each  arm  by  a  brass  rod  ; 
and  a  brass  ball  suspended  by  a  silk  cord  from  the 
ebonite  connector  hangs  between  them. 

As  each  bell  is  at  the  same  potential  as  the  jar  with 
which  it  is  connected,  the  ball  is  alternately  attracted 
and  repelled,  causing  the  bells  to  ring. 

Instruments  of  this  kind  have  no  practical  use,  except 
to  illustrate  the  principles  of  the  science. 

c  APPARENT  TIME  OF 

THE  ELECTRIC  DIS- 
CHARGE AN  OPTICAL 
ILLUSION. —  The  car- 
riers on  the  revolving 
B  plate  of  a  Topler  afford 
special  facilities  for 
this  experiment.  They 
are  usually  six  discs, 
arranged  in  a  circle, 
and  present  the  ap- 
pearance of  a  con  tin- 
Fig.  41-Chime  for  Topler  Machine.  ^^  ^^  ^  ^^ 

the  machine  is  operated  in  the  light ;  but  when  oper- 
ated in  the  dark,  they  are  seen  only  when  the  spark 
renders  them  visible ;  and,  instead  of  the  bright  ring, 
each  appears  by  itself,  apparently  motionless,  and  as 
perfect  in  form  as  if  really  so,  just  as  if  the  movement 
of  the  plate  were  momentarily  arrested  during  the 
passage  of  the  spark. 

This  apparent  time  of  the  spark  may  be  estimated 
at  i  second ;  but  if  the  carriers  were  really  visible 
during  that  time,  the  ring-like  appearance  would  be 


EXPERIMENTS  WITH  THE  TOPLER  MACHINE.       127 

unavoidable,  as  will  appear  from  the  following  calcu- 
lation. 

Suppose  the  revolving  plate  to  have  an  average  speed 
of  4 1  revolutions  per  second,  it  is  evident  that  each 
carrier  would  make  a  complete  revolution  in  less  than 
i  second  ;  consequently  if  that  were  the  actual  duration 
of  the  spark,  each  would  be  continuously  visible  round 
the  entire  circle,  and  hence  even  a  single  carrier  would 
produce  the  bright  ring.  But  it  is  only  necessary  to 
this  result  that  each  should  be  visible  until  it  takes  the 
place  of  its  predecessor — that  is  during  its  passage  of  J 
of  the  circle,  which  reduces  the  time  to  aV  of  &  second. 

But  if  they  were  visible  even  half  that  time,  -^  of  a 
second,  and  each  were  1J  inches  in  diameter,  and  their 
distance,  from  center  to  center,  6  inches,  we  would 
have  6  ellipses,  each  having  a  length  equal  to  twice  its 
breadth. 

From  this  it  is  evident  that  the  smallest  conceivable 
duration  of  spark  must  produce  an  ellipse :  but  as  each 
presents  the  appearance  of  a  circle,  with  no  tendency 
to  elliptical  form,  the  conclusion  is  inevitable  that  the 
apparent  duration  of  the  spark  is  an  optical  illusion, 
and  that  its  time  is  so  nearly  zero,  that  it  cannot  be 
estimated. 

We  must  conclude,  then,  that  at  the  instant  of  dis- 
charge the  image  of  the  carrier  is  photographed  on  the 
retina  of  the  eye,  and  at  the  next  instant  darkness 
supervenes :  but  the  sensation  on  the  retina  has  a  mo- 
mentary duration,  during  which  the  carrier  appears 
stationary,  while  in  reality  it  may  have  passed  entirely 
round  the  circle. 

It  is  important  to  notice,  in  this  connection,  that  the 
appearance  and  disappearance  of  the  carriers  depend 


128  ELEMENTS  OF  STATIC  ELECTRICITY. 

on  the  rapidity  of  the  discharge ;  and  when  the  spark 
is  made  so  short  and  rapid  as  to  be  apparently  contin- 
uous, the  carriers  appear  and  disappear  with  each  snap, 
like  a  succession  of  views  in  a  rapidly  moving  panora- 
ma, proving  that  the  apparently  continuous  spark  is  a, 
succession  of  sparks  so  rapid  as  to  give  the  impression 
of  continuity. 

As  a  flash  of  lightning  is  only  the  same  thing  on  a 
grander  scale  in  nature's  own  laboratory,  we  must  con- 
clude that  the  passage  of  electricity  from  cloud  to  cloud, 
a  distance  often  of  many  miles,  is  so  rapid  as  to  defy 
human  calculation.  We  notice  this  in  chain  lightning, 
when  the  flash,  sometimes  three  to  five  miles  long,  is 
seen  throughout  its  entire  length  at  the  same  instant, 
as  if  suddenly  photographed  on  the  cloud. 

TRANSMISSION  OF  POWER  BY  STATIC  ELECTRICITY. 
— Two  machines  are  necessary  for  this  experiment — 
one  called  the  primary,  and  the  other  secondary.  The 
secondary  should  be  a  very  light  running  machine  ; 
hence  it  is  better  to  make  it  smaller  than  the  primary, 
and  the  driving  wheel  and  switch  may  be  dispensed  with. 

Let  the  machines  be  placed  near  each  other,  in  the 
same  relative  position,  the  secondary  in  front ;  and 
connected  together  by  conducting  cords  or  wires,  joining 
similar  pairs  of  Leyden  jars:  and  let  the  sliding  elec- 
trodes be  separated  beyond  sparking  distance.  Now 
let  the  primary  machine  be  put  in  operation,  and  the 
movable  plate  of  the  secondary  will  rotate  in  a  direction 
opposite  to  that  of  the  primary.  If  the  electric  energy 
should  not  be  sufficient  to  overcome  the  friction  and 
inertia,  in  starting,  the  plate  of  the  secondary  may  be 
put  in  rotation  by  hand,  and  its  motion  will  then  be 
sustained  by  the  electric  action. 


EXPERIMENTS  WITH  THE  TOPLER  MACHINE.        129 

The  explanation  is  as  follows.  When  a  Topler  ma- 
chine is  in  operation,  there  is  a  strong  attraction  be- 
tween the  plates,  the  result  of  induction  from  the 
opposite  electric  states  of  the  parts  in  proximity.  This 
attraction  which  constantly  increases  up  to  the  instant 
of  discharge,  acts  as  a  resisting  force  which  must  be 
overcome  by  the  force  used  to  rotate  the  plate.  Now, 
when  the  two  machines  are  connected,  this  electric 
force  is  transmitted  to  the  secondary,  where,  having  no 
mechanical  force  to  oppose  it,  as  in  the  primary, 
it  causes  the  rotation  of  the  plate  in  the  opposite 
direction. 

Thus  the  mechanical  force  in  the  primary  is  trans- 
muted into  electric  force,  passes  <fver  to  the  secondary 
and  reproduces  mechanical  force ;  the  force  applied  to 
the  primary  being  expended  in  the  secondary. 

The  apparatus  thus  becomes  a  scientific  bank,  with 
its  receiving  and  paying  tellers.  But  nature  is  a 
shrewd  banker,  and  always  exacts  full  discount ;  hence 
the  mechanical  energy,  paid  in  to  the  primary,  is  dis- 
counted by  friction,  leakage,  and  heat;  so  that  the 
remaining  energy  may  not  be  sufficient  to  start  the 
plate  of  the  secondary  into  rotation  without  an  ad- 
ditional payment. 

The  sliding  electrodes  in  the  secondary  machine  may 
l)e  adjusted  to  produce  the  electric  discharge  with  spark 
and  snap,  instead  of  the  mechanical  rotation  of  the 
plate ;  thus  illustrating  the  transmutation  of  force,  at 
will,  from  mechanjcal  to  electric,  and  from  electric 
either  back  again  to  mechanical,  or  to  the  heat,  light, 
and  sound  of  the  electric  discharge. 

SOUECE  OF  ELECTRIC  SUPPLY  OF  THE  TOPLEK  MA- 
CHINE.— The  earth,  the  machine  itself,  and  the  air  are 


130  ELEMENTS  OF  STATIC  ELECTRICITY. 

the  only  sources  from  which  an  electric  machine  can 
derive  electricity. 

With  the  common  friction  machine  a  connection 
with  the  earth  is  indispensable,  and  only  a  very  limited 
charge  can  be  obtained  without  it ;  the  transfer  being 
either  from  the  earth  to  the  machine,  or  from  the  ma- 
chine to  the  earth,  as  explained  on  page  99.  Hence  it 
is  often  compared  to  a  pump,  drawing  electricity  from 
the  earth  through  a  chain.  Remove  the  chain  and  the 
supply  ceases. 

But  with  the  Topler  a  similar  earth  connection 
diminishes  the  charge ;  showing  a  loss  instead  of  an 
increase  of  charge.  Indeed,  perfect  insulation  of  the 
generating  parts  is  an  essential  feature  of  the  machine. 

To  demonstrate  this  more  perfectly,  let  the  machine 
be  put  in  operation  on  an  insulated  platform,  when  it 
will  be  found  that  there  is  not  the  slightest  perceptible 
diminution  of  electric  energy.  It  is  evident,  then,  that 
the  earth  is  not  its  source  of  supply. 

A  certain  amount  is,  no  doubt,  obtained  from  the 
material  of  the  machine  itself;  but  this  source  would 
soon  be  exhausted  by  such  experiments  as  the  charg- 
ing of  a  large  Leyden  battery ;  whereas  such  a  battery 
may  be  charged  without  diminishing  the  energy  of  the 
machine. 

The  air,  then,  is  the  only  remaining  source,  and  the 
large  amount  of  ozone  generated  by  this  machine  is 
conclusive  evidence  of  its  electro-chemical  action  on 
the  air,  and  strong,  presumptive  evidence  that  the  air, 
thus  acted  upon,  has  furnished  the  electricity  whose 
action  has  changed  the  oxygen  to  ozone. 

This  would  imply  that  ozone  is  the  result  of  depriv- 
ing air  of  a  portion  of  its  electricity ;  whereas  if  the 


EXPERIMENTS  WITH  THE  TOP  LEE  MACHINE.        131 

electricity  were  derived  from  the  earth,  we  must  infer 
that  its  generation  precedes  the  generation  of  ozone, 
instead  of  being  coincident  with  it.  But  the  insulation 
proves  that  the  earth  does  not  supply  the  electricity ; 
so  that  the  weight  of  evidence  is  in  favor  of  ozone  being 
the  direct  result  of  electric  generation,  rather  than  a 
result  of  subsequent  electric  action.  And,  if  such  is 
the  case,  it  is  strong  proof  that  the  air  is  the  chief 
source  of  electric  supply. 

The  generation  of  ozone  by  atmospheric  electricity 
during  thunder  storms  is  a  well-known  fact;  and  clouds, 
floating  miles  above  the  earth,  must  obtain  their  elec- 
tricity either  from  their  own  vapor,  or  the  air,  or  both. 
Such  clouds,  at  different  electric  potentials,  insulated 
from  the  earth,  acting  inductively  on  each  other,  and 
finally  producing  a  discharge,  fulfill  the  same  conditions 
as  exist  in  the  Topler  machine ;  and  the  generation  of 
ozone  is  doubtless  due  to  the  same  cause  in  both.  And 
since  the  vapor  of  the  cloud  corresponds  to  the  material 
of  the  machine,  and  it  has  been  shown  that  the  electric 
supply  of  the  machine  from  its  own  material  must  be 
very  limited ;  and  since  the  machine  operates  most 
effectively  in  a  dry  atmosphere,  and  hence  does  not 
derive  its  electricity  from  vapor ;  we  may  infer  that  the 
electric  action  is  the  same  in  both  cases,  and  that  the 
air  is  the  chief  source  of  electric  supply. 

It  is  evident  from  the  movement  of  particles  of  dust 
and  other  light  bodies  towards  the  machine,  that  the 
air  in  which  these  atoms  float  must  have  a  similar 
movement ;  that  currents  of  air  are  constantly  flowing 
to  the  machine  and  that  this  air,  after  being  changed 
to  the  same  electric  potential,  is  repelled,  and  air  at  a 
different  potential  flows  in  to  take  its  place ;.  a  move- 


132  ELEMENTS  OF  STATIC  ELECTRICITY. 

merit  similar  to  that  which  takes  place  in  the  hot  and 
cold  currents  round  a  heated  stove. 

But  the  initial  charge  is  undoubtedly  from  the  ma- 
terial of  the  machine  itself,  and  results  from  the  friction 
of  the  brushes  on  the  carriers ;  after  which  follows  the 
increase  by  induction  and  the  action  on  the  air. 

ELECTRICITY  GENERATED  BY  THE  FRICTION  OF 
METALS. — The  old  division  of  all  substances  into  elec- 
trics and  non-electrics  was  the  exponent  of  the  idea 
then  prevalent,  that  only  certain  substances,  as  glass, 
sealing-wax,  and  other  non-conductors,  comprised  in  a 
very  brief  list,  were  capable  of  electric  excitation. 
While  this  view  is  no  longer  maintained,  yet,  since  in 
nearly  all  experiments  illustrating  the  elements  of  static 
electricity,  glass,  sealing-wax,  ebonite,  silk,  wool,  fur, 
and  other  non-conductors,  are  almost  exclusively  em- 
ployed as  generators,  we  are  apt  to  lose  sight  of  the 
fact  that  metals  and  other  conductors  are  capable  of 
generating  electricity  by  their  mutual  friction.  And 
yet  this  is  one  of  the  most  important  principles  of  static 
electricity.  It  is  that  which  liberates  our  ideas  of  elec- 
tricity from  the  narrow  bounds  to  which  they  were 
once  confined,  proving  that  it  is  not  a  special  property 
of  certain  substances,  but  a  universal  property  of  mat- 
ter, one  form  of  that  energy  which  pervades  and  con- 
trols the  universe. 

This  point  has  been  already  illustrated,  but  the 
Topler  machine  affords  special  facilities  for  illustrating 
it  more  fully.  In  it  the  initial  charge  is  produced  by 
the  friction  of  metal  brushes  on  metal  carriers.  True, 
both  carriers  and  brushes  are  attached  to  glass,  and  the 
glass  subsequently  acts  by  induction  as  a  generator; 
but  the  friction  is  confined  to  the  carriers  and  brushes 


EXPERIMENTS  WITH  THE  TOPLER  MACHINE.        133 

alone;  and,  so  far  as  the  electricity  is  obtained  from 
this  source,  the  glass  acts  only  as  an  insulator  to  pre- 
vent the  escape  of  the  electricity  generated  by  the 
friction,  from  which  the  initial  charge  is  derived. 

It  is  not  even  necessary  that  the  metals  should  be 
different.  The  machines  here  described  are  constructed 
with  brass  carriers,  and  brushes  of  brass  wire;  and, 
though  the  carriers  are  nickel-plated,  so  that  the  friction 
is  that  of  brass  brushes  on  a  nickel  surface,  yet  carriers 
left.unplated  give  equally  as  good  results. 

THE  SPARK;  ITS  DIRECTION,  SUBDIVISION,  AND 
COLOR. — The  spark  from  a  Topler  machine  presents 
phenomena  which  demand  careful  investigation. 

As  already  shown,  the  apparent  time  of  the  discharge 
is  an  optical  illusion,  time  being  practically  annihilated; 
so  that  it  is  impossible,  from  observation,  to  tell  in  what 
direction  the  discharge  takes  place.  A  brilliant  streak 
of  white  light,  extending  from  one  electrode  to  the  other, 
suddenly  appears  and  disappears,  leaving  us  in  igno- 
rance as  to  the  direction  in  which  it  moves.  But  the 
following  experiment  affords  better  opportunity  for 
observation. 

As  already  shown,  the  electric  connection  between  the 
inside  coatings  of  the  Leyden  jars  may  be  interrupted 
by  separating  the  sliding  electrodes,  and  that  between 
their  outside  coatings  by  opening  the  switch.  Put  the 
machine  in  operation  in  a  darkened  room  at  night,  with 
the  switch  open,  arid  the  sliding  electrodes  separated 
three  or  four  inches.  From  the  electrode  P,  Fig.  42, 
a  brush  of  violet -colored  light,  diverging  from  a  small, 
circular  space,  extends  about  f  of  an  inch  towards  the 
opposite  electrode,  accompanied  by  a  hissing  sound. 
The  opposite  electrode,  R,  remains  comparatively  qui- 


134  ELEMENTS  OF  STATIC  ELECTRICITY. 

escent  at  first,  showing  only  a  glow  of  light ;  but,  as 
the  electricity  accumulates,  there  is  a  sudden  outburst 
from  it,  accompanied  by  phenomena  of  the  most  inter- 
esting and  varied  character. 

A  brush  of  light,  of  a  faint  white,  or  violet  color, 
darts  across  the  intervening  space,  diverging  towards 
the  center,  and  converging  as  it  meets  the  brush  from 
the  opposite  electrode  ;  forming  an  elliptical  figure,  two 
or  three  inches  in  diameter,  extending  from  one  elec- 
trode to  the  other.  Through  the  center  of  this  brush 
shoot  out  long  tongues  of  red  and  violet  light,  curving 
and  branching  in  a  variety  of  fantastic  forms.  Some- 
times five  or  six  of  these  appear  at  once,  like  fiery 
serpents,  hissing,  spitting,  and  darting  out  their  red 
forked  tongues.  Sometimes  the  appearance  is  that  of 
a  miniature  tree,  its  main  trunk  branching  off  at  various 
angles  and  curves.  Then,  again,  the  brush  disappears, 
and  we  have  a  single,  straight,  violet  colored  stem, 
about  |  of  an  inch  long,  which  divides  into  a  great 
number  of  bright  rays,  radiating  in  straight  lines  from 
the  end  of  the  stem,  and  forming  a  globe  of  white  light, 
about  three  inches  in  diameter :  the  whole  resembling 
a  little  bush  of  remarkably  regular  appearance,  in 
marked  contrast  with  the  curved  and  contorted  phe- 
nomena just  described. 

Between  this,  and  the  short  brush  on  the  opposite 
electrode,  a  dark  space  intervenes,  into  which  the  rays 
pass  and  intermingle ;  the  brush  from  the  electrode  R 
being  largely  in  excess  of  the  other,  and  showing  far 
greater  energy ;  but  more  fitful,  coming  at  first  in  jets, 
with  a  spitting  sound,  while  the  other  is  more  constant, 
with  a  steady,  hissing  sound. 

As  explained  on  page  118,  electric  movement  is  from 


EXPERIMENTS  WITH  THE  TOPLER  MACHINE.        135 

the  comb  L  downwards  into  the  jar  D,  then  along  the 
switch,  when  closed,  and  its  connections  to  (7,  and  up- 
ward out  of  C  to  the  plate  and  carriers;  and,  in  part, 
along  the  electrode  P  towards  72,  which  connects  with 
the  inside  of  D:  the  glass  of  the  jars,  and  the  air  space 
between  P  and  R  forming  barriers  at  which  electricity 


42— Experiments  with  the  Topler  Electric  Machine. 


accumulates  on  the  side  towards  which  the  movement 
takes  place,  induction  producing  a  corresponding  neg- 
ative on  the  opposite  side. 

D  has  the  higher  potential,  as  already  shown,  but  its 
inside  charge  is  bound  by  an  equal  negative  on  its 
outer  coating;  while  electricity  is  repelled  from  the 


130  ELEMENTS  OF  STATIC  ELECTRICITY. 

inside  of  C.  Hence,  when  the  switch  is  open,  we  have 
the  difference  in  the  brush  discharge  already  described. 

But  as  the  higher  charge  of  D  continues  to  accumu- 
late on  its  inside  coating,  the  tension  increases  on  the 
electrode  R,  till  the  electricity  finally  bursts  through 
the  resisting  air  from  R  to  P ;  producing  the  spark  and 
snap  when  the  switch  is  closed,  as  already  explained. 

The  effect  of  opening  the  switch  is  to  substitute  for 
this  metal  conductor,  which  has  comparatively  no 
resistance,  a  portion  of  the  base,  which  is  of  kiln- 
dried  wood,  and  offers  high  resistance.  This  retards 
the  current,  producing  the  difference  of  phenomena 
between  the  bright,  instantaneous  spark  of  white  light, 
with  its  sharp  report,  and  the  slow  moving  brushes  of 
violet  light,  with  their  hissing,  spitting  sounds;  and 
from  this  slow  movement  we  are  able  to  determine  the 
direction  of  the  discharge,  as  already  shown. 

The  cause  of  the  subdivision  of  the  spark  when  the 
switch  is  open  next  claims  attention.  It  has  been 
shown  that  the  discharge  between  the  inside  coatings 
through  the  electrodes  P  and  72,  and  the  intervening 
air  space,  is  dependent  on  the  counter  discharge 
between  the  outside  coatings,  through  the  switch,  when 
closed,  or,  through  the  kiln-dried  wood  of  the  base, 
when  the  switch  is  open.  This  discharge  may  be  seen 
by  opening  the  switch,  half  an  inch  or  more,  so  that 
the  resistance  of  the  air  is  less  than  that  of  the  wood. 
We  then  have  a  bright  spark  below,  simultaneous  with 
the  spark  above.  But  when  the  switch  is  opened  so 
that  the  resistance  of  the  air  is  greater  than  that  of  the 
wood,  the  discharge  below  takes  place  silently  through 
the  wood,  and  we  have  above,  the  subdivided,  colored 
discharge  already  described. 


EXPERIMENTS  WITH  THE   TOPLER  MACHIN         ^1 ' 

• 

Witli  the  switch  closed,  reducing  the  resistance  Below 
to  zero,  the  discharge  through  the  air  is  instantaneoufc^V/ 
and  there  is  seldom  any  subdivision,  except  that  a  long 
spark  from  a  heavy  charge  sometimes  divides  into  two, 
slightly  separated  during  a  part  of  their  course.  But, 
with  the  switch  open,  the  high  resistance  retards  the 
lower  discharge,  which  is  compelled  to  force  its  way 
slowly  through  the  kiln-dried  wood;  making  the 
change  of  potential  between  the  outside  coatings  slow 
and  gradual,  and  producing  a  similar  effect  on  the 
inside  coatings.  Now,  as  the  spark  is  caused  by  the 
electricity  forcing  its  way  through  the  air,  whose  elec- 
trified molecules  are  at  the  same  potential  near  each 
electrode,  and  hence  self-repellent,  while  the  surround- 
ing air  is  at  a  lower  potential  and  attractive,  these 
forces,  acting  in  part  at  right  angles  to  the  original 
impulse,  during  the  comparatively  slow  progress  of 
the  discharge,  produce  the  brushes  of  diverging  rays 
already  described.  Various  influences,  such  as  currents 
of  air,  particles  of  dust,  and  the  induction  of  electricity 
generated  on  adjacent  parts  of  the  machine,  curve  and 
contort  the  spark,  producing  the  peculiar  phenomena 
already  described  in  connection  with  the  brushes,  and 
also  affecting  the  long  blight  sparks  in  a  similar  manner. 

We  next  notice  the  color  of  the  spark.  Light  is  a 
mode  of  motion,  and  its  color  is  influenced  by  the 
intensity  of  the  motion.  A  bar  of  iron,  dra.wn  from  the 
furnace,  ready  for  rolling  or  welding,  is  said  to  be  at  a 
white  heat;  as  it  cools  it  changes  to  a  red  heat.  Here 
the  color  of  the  light  depends  on  heat,  which  is  also  a 
mode  of  motion ;  and  as  the  intensity  of  the  heat  mo- 
tion decreases,  the  light  changes  from  white  to  red  of 
various  shades,  till  the  bar  resumes  its  original  color. 


138  ELEMENTS  OF  STATIC  ELECTRICITY. 

The  brilliancy  of  the  arc  in  the  electric  lamp  is  due 
to  the  intensity  of  the  motion,  while  the  softer  light  of 
the  incandescent  lamp  results  from  a  motion  less  intense. 
When  an  electric  lamp  is  being  lighted  or  extinguished, 
the  change  of  color  from  white  to  the  various  shades  of 
red  is  evidently  dependent  on  decrease  of  motion. 
Must  we  not  conclude  then  that  the  white  light  of  the 
electric  spark,  when  the  switch  is  closed,  is  due  to 
intensity  of  motion,  and  the  colored  light  with  the  open 
switch,  to  decrease  of  intensity,  as  in  the  iron  bar  or 
the  carbon  of  the  electric  lamp  ?  Or,  if  light  and  heat 
are  modes  of  motion,  is  not  the  evidence  equally  strong 
that  electricity  is  a  mode  of  motion?  Or  may  we  not 
go  still  farther,  and  say  that  light,  heat,  and  electricity 
are  only  different  manifestations  of  that  energy  which 
is  a  universal  property  of  all  matter,  of  which  the  ex- 
periments here  given  are  an  additional  proof?  For  in 
the  electric  spark,  we  have  light,  heat,  and  electricity 
combined. 

Having  stated  that  D  is  usually  the  jar  of  higher 
potential,  it  should  be  explained,  that  there  is  fre- 
quently a  temporary  reversal  of  potential;  and,  when 
this  occurs,  all  the  phenomena  here  described  are 
reversed  also.  The  cause  of  this  reversal  will  be  ex- 
plained in  connection  with  the  following  experiment. 

DIRECT  AND  REVERSED  ROTATION. — A  Topler  ma- 
chine can  be  charged  only  by  revolving  the  smaller 
plate  in  a  given  direction;  which,  in  the  machine 
represented,  is  shown  by  the  arrow. 

The  reason  is  this:  In  order  to  store  up  electricity 
in  the  Leyden  jars,  each  carrier  must  pass  from  an 
insulated  brush,  where  it  is  charged,  directly  to  a  comb 
connecting  with  a  Leyden  jar,  before  it  passes  to  an 


EXPERIMENTS  WITH  THE  TOPLER  MACHINE.        139 

uninsulated  brush,  where  it  is  discharged.  Thus  the 
carrier  TF,  charged  by  the  friction  of  the  insulated 
brush  E,  must  pass  to  the  comb  Z,  connecting  with  the 
jar  D,  and  give  up  its  principal  charge,  before  passing- 
to  the  uninsulated  brush  and  comb  H,  where  its  resid- 
ual is  discharged  through  the  brass  rod  H  F",  which 
puts  it  in  electric  connection  with  the  carrier  Z,  of 
opposite  potential.  Reverse  the  rotation,  and  the 
carrier  TF,  starting  from  E^  would  give  up  its  princi- 
pal charge  to  the  uninsulated  brush  and  comb  at  F~, 
before  reaching  the  comb  K,  connecting  with  the 
Ley  den  jar  (7,  where  only  the  residual  would  remain. 
It  must  also  be  noticed  that  the  charge  is  greatly 
increased,  both  on  the  carrier  and  adjacent  portion  of 
the  plate,  by  passing  the  inductor  T,  attached  to  the 
stationary  plate  B ;  whereas,  when  the  rotation  is  re- 
versed, the  carrier  leaves  the  inductor  and  passes  the 
space  between  T  and  X,  where  the  induction  is  almost 
zero.  Thus  it  is  evident  that  no  storage  of  electricity 
in  the  Leyden  jars,  and  hence  no  permanent  charge  can 
be  obtained  from  a  reversed  rotation. 

HIGHER  POTENTIAL  OF  JAR  D. — It  has  been  shown, 
that  from  the  higher  position,  and  hence  better  insula- 
tion of  the  brush  E,  and  upper  half  of  the  revolving 
plate  .A,  as  compared  with  the  lower  position,  and  con- 
sequent inferior  insulation  of  the  brush  F,  and  lower 
half  of  A,  the  potential  of  the  jar  .Z),  receiving  its 
charge  from  the  former,  must  be  higher,  as  a  rule,  than 
that  of  C,  which  receives  its  charge  from  the  latter. 

Repeated  experiments,  made  by  the  author  with  a 
number  of  different  machines  of  this  kind,  fully  confirm 
this  view.  The  higher  potential  is  shown  by  the  fre- 
quent partial  discharges  between  the  inside  and  out- 


140  ELEMENTS  OF  STATIC  ELECTRTCITY. 

side  coatings  of  this  jar ;  and,  in  case  of  fracture,  which 
sometimes  occurs  from  an  overcharge,  it  is  always  this 
jar  which  is  broken :  and  the  fracture  always  occurs  on 
the  side  nearest  the  opposite  jar,  showing  that  the 
charge  is  attracted  to  that  side,  and  electricity  repelled 
from  the  outside  coating,  creating  a  sufficient  difference 
of  potential  between  the  two  coatings  to  overcome  the 
resistance  of  the  glass  and  perforate  it. 

REVEKSAL  OF  POTENTIAL. — It  has  been  already 
stated  that  there  is  frequently  a  temporary  reversal  of 
potential.  Such  a  reversal  can  be  produced,  if  desired, 
by  joining  the  electrodes  P  and  72,  and  reversing  the 
rotation  of  the  plate  till  the  machine  is  fully  discharged; 
then  separating  P  and  R  while  the  reversed  rotation  is 
continued,  and  then  resuming  the  direct  rotation,  when 
a  complete  reversal  of  potential  will  be  found  to  have 
occurred,  which  will  continue  till  again  reversed  by  a 
similar  experiment,  or  till  the  machine  has  had  a  period 
of  rest.  The  explanation  is  as  follows:  — 

When  P  and  R  are  separated,  and  the  rotation  re- 
versed, the  same  causes  which  before  operated  to  raise 
the  potential  of  D  above  that  of  67,  now  operate  to  raise 
the  potential  of  O  above  that  of  7),  but  in  a  very  lim- 
ited degree.  For,  as  already  shown,  any  carrier,  as  W, 
charged  by  the  brush  E,  would  now  give  up  its  princi- 
pal charge  to  the  brush  and  comb  at  V;  but  the  resid- 
ual, slightly  increased  by  the  inductor  X,  would  be 
given  up,  through  the  comb  TTto  the  jar  O;  while  the 
opposite  carrier  Z,  would  give  up  its  principal  charge 
at  H,  and  carry  its  residual  to  the  comb  L,  and  the  jar 
Z>,  after  a  slight  increase  by  the  inductor  T.  But  the 
difference  of  insulation  between  the  upper  and  lower 
parts  affect  these  residual  discharges  in  the  same  man- 


EXPERIMENTS  WITH  THE  TOPLER  MACHINE.        141 

ner  as  the  principal  discharges,  and  hence  operate  to 
make  the  potential  of  6Y,  receiving  its  charge  from 
above,  higher  than  that  of  Z>,  receiving  its  charge  from 
below.  This  residual  is  not  sufficient  of  itself  to  bring 
the  machine  into  action,  but  it  creates  a  slight  differ- 
ence in  favor  of  (7,  sufficient  to  sustain  a  reversal  of 
potential  when  the  direct  rotation  is  resumed. 

THE  F  ARABIC  CURRENT. — The  faradic  current  con- 
sists of  a  series  of  electric  impulses  following  each  other 
with  great  rapidity.  It  is  obtained  from  the  battery 
and  coil  by  a  spring  vibrator,  which  opens  and  closes 
the  circuit;  and  from  the  magneto-electric  machine  by 
a  revglving  electro-magnet  and  commutator. 

Both  these  instruments  have,  for  many  years,  been 
extensively  used  in  medical  practice ;  but  the  use  of  a 
static  machine  for  this  purpose  is  quite  recent,  and  the 
switch,  on  the  machine  here  represented,  affords  special 
facilities  for  producing  and  utilizing  this  current.  In 
Fig.  42  are  shown  two  sockets,  on  the  front  edge  of 
the  base,  connecting  with  the  terminals  of  the  switch, 
into  which  are  inserted  the  tips  of  conducting  cords,  to 
the  outer  extremities  of  which  may  be  attached  metal 
handles,  as  shown,  or  other  electrodes  suitable  for  the 
use  of  this  current,  for  medical  or  scientific  purposes. 

As  already  explained,  when  the  machine  is  in  oper- 
ation there  is  a  constant  movement  of  electricity 
through  the  switch  and  its  connections,  from  1)  to  (7, 
while  the  charge  is  accumulating;  and  the  counter  dis- 
charge through  them,  from  0  to  D,  is  simultaneous  with 
the  discharge  above,  from  R  to  P.  When  the  switch  is 
open  and  the  cords  attached,  as  shown,  this  discharge 
must  either  force  its  way  through  the  kiln-dried  wood, 
or  pass  out  through  the  cords  and  any  object  connected 


142  ELEMENTS  OF  STATIC  ELECTRICITY. 

with  their  outer  terminals,  according  to  the  degree  of 
resistance  offered  by  each  path  respectively.  If  a  per- 
son, or  a  number  of  persons  with  hands  joined,  grasp 
the  handles,  the  resistance  will  be  less  than  through  the 
wood,  and  they  will  feel  the  effects  of  the  discharge. 
This  discharge  is  regulated  by  the  distance  to  which  R 
and  P  are  separated.  With  a  separation  of  TV  of  an 
inch,  on  a  large  machine,  the  discharge  is  so  rapid  that  the 
distinction  between  the  impulses  can  scarcely  be  per- 
ceived ;  producing  a  faradic  current  smoother  than  can 
be  obtained  from  the  best  batteries,  while  a  separation 
of  i  an  inch  produces  effects  which  the  strongest  nerves 
cannot  endure. 

This  current,  in  its  milder  form,  cannot  be  distin- 
guished from  that  obtained  from  the  battery,  or  mag- 
neto-electric machine :  but,  in  its  more  powerful  effects, 
it  is  more  impulsive ;  coming  in  jets,  with  cumulative 
force,  like  the  rapid  blows  of  a  planishing  hammer.  In 
the  battery  current,  the  stronger  effects  show  increased 
intensity,  and  a  greater  tendency  to  muscular  contrac- 
tion ;  while  increase  of  strength  in  this  current  is 
due  to  the  slower  impulses  giving  more  time  for  the 
accumulation  of  electric  energy. 

THE  ELECTRIC  BATH  AND  ELECTRIC  WIND. — Charg- 
ing a  person  on  an  insulated  stool  is  one  of  the  most 
common  experiments  in  static  electricity,  but  it  has 
only  recently  come  into  use  in  medical  practice;  and, 
instead  of  the  stool,  an  insulated  platform,  on  which 
one  or  more  persons  can  be  comfortably  seated,  has 
been  substituted;  the  treatment  being  known  as  the 
"Electric  Bath." 

When  the  patient  is  seated,  as  above,  the  electrodes 
P  and  R,  drawn  out  beyond  sparking  distance,  and  the 


EXPERIMENTS  WITH  THE  TOPLER  MACHINE.        143 

switch  closed,  a  connection  is  made  between  the  pa- 
tient and  the  machine  by  a  conducting  cord;  one  end 
being  attached  to  the  ball  surmounting  one  of  the 
Leyden  jars,  and  the  other  end  to  the  chair.  A  similar 
connection  is  made  between  the  opposite  jar  and  the 
floor  near  thg  platform,  to  create  a  certain  degree  of 
induction,  and  so  facilitate  the  process  of  charging, 
which  is  now  done  by  putting  the  machine  in  oper- 
ation. Very  little  sensation  is  experienced  from  this 
charge,  but  its  effect  in  certain  nervous  diseases,  which 
cannot  be  treated  with  the  battery,  such  as  St.  Vitus 
dance,  is  said  by  medical  men  to  be  very  soothing.  In 
other  cases,  sparks  are  drawn  from  the  patient  with  the 
hand  or  a  suitable  electrode,  as  a  ball,  roller,  or  sponge, 
attached  to  the  cord  from  the  opposite  jar,  and  held  by 
an  insulating  handle. 

The  electric  wind  is  given  by  a  point  electrode, 
attached  as  above,  either  with  or  without  the  insulated 
platform.  A  gentle  current  of  electrified  air  from  the 
point  fans  the  patient,  producing  a  delightfully  sooth- 
ing sensation. 

Electric  treatment  of  this  kind  can  be  given  only  by 
static  electricity,  and  its  value  must  be  determined  by 
the  medical  profession,  among  whom  it  is  coming  into 
favor;  being  used  and  recommended  by  physicians  of 
eminence. 

GAS  LIGHTING. — Lighting  the  gas  in  churches  and 
public  halls  by  electricity  is  commonly  done  by  a  bat- 
teiy  and  coil,  but  the  Topler  machine  can  also  be  used 
for  this  purpose.  With  either  method  there  must  be 
wires  connecting  the  generator  with  the  chandeliers, 
wires  connecting  the  chandeliers  together,  and  also  the 
separate  burners;  all  arranged  in  one  circuit  and  prop- 


144  ELEMENTS  OF  STATIC  ELECTRICITY. 

erly  insulated.  At  each  burner  there  is  a  break  in  the 
circuit,  so  arranged  that  a  short  spark  will  pass  through 
the  gas;  the  ends  of  the  wire  being  attached  to  an 
insulator  fitted  to  the  burner. 

With  the  battery  there  is  a  ground  wire,  and  con- 
nection with  the  gas  pipe  to  complete  the  circuit;  but, 
with  the  machine,  the  circuit  is  made  by  two  separate 
wires,  connecting  the  chandeliers  with  the  balls  sur- 
mounting the  Leyden  jars.  On  account  of  the  greater 
intensity  of  static  electricity,  these  wires  must  be  thor- 
oughly insulated  with  thick  rubber  tubing,  wherever 
they  are  liable  to  come  in  contact  with  the  walls  or  gas 
fixtures.  With  these  arrangements  properly  made,  it 
is  only  necessary  to  close  the  switch,  separate  P  and  R, 
to  the  full  extent,  turn  on  the  gas,  and  put  the  machine 
in  operation.  The  resistance  of  the  air  between  P  and 
R,  being  greater  than  the  resistance  of  the  wires  and 
the  short  breaks  between  their  terminals  at  the  burners, 
the  sparks  take  place  at  the  burners,  and  the  gas  is  lit. 

As  to  the  expense,  convenience,  and  efficiency  of  this 
system,  as  compared  with  the  battery  system,  only  gen- 
eral statements  can  as  yet  be  made.  The  first  cost 
would  probably  be  about  the  same;  after  which  there 
would  be  no  further  expense  with  the  machine,  which, 
with  proper  care,  should  remain  in  good  working  order, 
for  this  purpose,  for  an  indefinite  term  of  years;  while 
the  battery  requires  frequent  renewal  of  the  fluid,  and 
occasional  renewal  of  the  zinc,  besides  cleansing  and 
amalgamating. 

As  to  efficiency,  the  greater  intensity  of  the  spark 
from  the  machine  will  be  evident,  when  we  consider 
that  a  machine  of  very  moderate  size  will  easily  pro- 
duce sparks  three  to  five  inches  in  length,  while  a  very 


EXPERIMENTS  WITH  THE  TOPLER  MACHINE.        145 

large  battery  and  coil  would  be  required  to  produce  the 
same  result.  But  this  should  be  taken  merely  as  an 
indication  of  comparative  intensity ;  as,  practically,  only 
very  short  sparks  are  required :  so  that  a  battery  and 
coil  of  medium  size  is  generally  sufficient. 

A  damp  atmosphere  does  not  affect  the  battery, 
while  it  lessens  the  energy  of  the  machine;  and,  in 
unskillful  hands,  may  interfere  with  its  practical 
efficiency.  But,  with  either  system,  the  person  in 
charge  should  have  a  thorough  knowledge  of  its  care 
and  management:  in  which  case  the  machine  can 
always  be  kept  in  practical  working  order. 


CHAPTER  X. 
ELECTEIC  TRANSMISSION  IN  VACUA. 

ELECTRIC  TRANSMISSION  IN  Low  VACUA. — Let  a 
glass  tube,  about  thirty  inches  long,  be  provided 
with  brass  caps  at  each  end,  fitting  air  tight;  from 
each  of  which  a  pointed  brass  rod  projects  inwards. 
And  let  a  stop-cock  be  attached  to  one  of  the  caps,  by 
which  the  tube  can  be  connected  with  an  air  pump,  as 
shown  in  Fig.  43. 

Let  the  tube  be  insulated,  and  the  caps  connected  by 
conducting  cords  with  the  balls  surmounting  the  Ley- 
den  jars  of  the  Topler  machine;  the  sliding  electrodes 
being  separated  to  their  full  extent.  When  filled  with 
air,  at  the  ordinary  atmospheric  density,  it  will  be  found 
impossible  to  pass  an  electric  charge  through  a  tube  of 
this  length:  but  let  it  be  connected  with  an  air  pump, 
and  the  air  well  exhausted,  and  a  charge  Avill  easily  pass 
through.  This  proves  that  air  at  the  ordinary  density 
has  a  much  higher  electric  resistance  than  rarefied  air. 

But  if  a  high  degree  of  vacuum  is  produced,  it  will  be 
found  much  more  difficult  to  pass  the  charge  through ; 
which  indicates  that  a  medium,  consisting  of  some 
material  substance,  is  essential  to  electric  existence 
and  movement;  and  that  if  it  were  possible  to  produce 
an  absolute  vacuum,  electricity  could  not  pass  through. 

If  the  above  experiment  be  performed  in  a  dark  room, 
flashes  of  red  and  violet  colored  light  will  be  seen  to 


ELECTRIC  TRANSMISSION  IN  VACUA.  147 

accompany  the  discharge,  strongly  resembling  the  cor- 
uscations of  the  aurora  polaris.  Hence  tubes,  used  for 
this  purpose,  have  been  called  aurora  tubes. 

GEISSLER  TUBES  — Improved  tubes  of  this 
kind,  called  from  their  inventor  Geissler  tubes, 
are  constructed  with  fine  platinum  wire  sealed 
into  their  extremities;  the  points  projecting 
inwards,  and  loops  formed  outside  for  the 
attachment  of  conducting  cords  or  wires.  The 
glass  is  bent  into  a  variety  of  graceful  curves 
and  folds:  small  tubes,  bent  in  this  manner, 
being  inclosed,  for  protection,  in  large  straight 
ones;  and  thus  long,  frail  tubes  are  reduced  to 
compact,  convenient  forms,  in  which  they  can 
be  safely  handled,  as  shown  in  Fig.  44. 

The  air  is  exhausted  from  them  by  a  mercury 
pump,  after  which  they  are  hermetically  sealed. 
The  expansion  of  the  fine  platinum  wires  being 
very  slight  and  nearly  the  same  as  that  of  the 
glass,  is  not  sufficient  to  cause  fracture,  hence 
the  vacuum  produced  in  well-constructed  tubes 
remains  permanent  for  years. 

Beautiful  fluorescent  effects  are  obtained  by 
constructing  such  tubes  of  uranium  glass.  Sim- 
ilar effects  are  also  obtained  by  introducing  into 
them  various  solids  and  gases;  as  sulphate  of 
quinine,  fluoride  of  boron,  fluoride  of  silicon, 
iodine,  hydrogen,  and  nitrogen ;  which  give 
certain  characteristic  colors,  when  subjected  to 
electric  action.  ^g.  43- 

The  effect  of  the  discharge  is  greatly  increased     Tube- 
if  a  break  be  made  in  the  connection  between  one  end 
of  the  tube  and  the  machine,  so  as  to  introduce  a  short 


148  ELEMENTS  OF  STATIC  ELECTRICITY. 


Fig.  44— Geissler  Tubes. 


ELECTRIC  TRANSMISSION -IN 


149 

ix</A0 
spark  into  the  circuit.    The  electricity  then  accumulate 

and  a  rapid  succession  of  brilliant  discharges  is  the  resul 

The  same  effect  can  be  produced  by  opening  the 
switch,  connecting  the  tube  with  its  terminals,  arid 
slightly  separating  the  sliding  electrodes  P  and  R. 

When  the  spark  between  P  and  R  is  apparently 
continuous,  the  pulsations  in  the  induced  discharge 
through  the  tube  are  distinctly  visible  in  the  alterna- 
tions of  light  and  shade;  proving  that  the  discharge 
consists  of  a  series  of  distinct  impulses. 


Fig.  45— Rotary  Movement  in  High  Vacua. 


ELECTRIC  TRANSMISSION  IN  HIGH  VACUA. — The  re- 
sidual air  in  the  ordinary  Geissler  tube  is  about  jo^An 
of  an  atmosphere,  but  Crookes  has  produced  tubes  in 
which  the  residual  is  less  than  Tutr&injtf  °f  an  atmos- 
phere ;  and  the  electric  discharge  in  such  tubes  presents 
certain  peculiarities  not  observed  in  ordinary  vacua. 

Electric  action  on  substances  inclosed  in  such  tubes, 
and  on  the  glass  itself,  is  increased  in  the  ratio  of  the  in- 
creased vacuum;  since  those  substances  receive  the  force 
of  energy  which,  in  lower  vacua,  is  expended  on  the  air. 

This  increased  action  is  shown  by  an  increase  in  the 
light  and  heat  developed  in  them,  and  in  the  attractive 


150  ELEMENTS.  OF  STATIC  ELECTRICITY. 

force  exerted  on  them,  as  shown  when  they  are  free  to 
move. 

Fig.  45  shows  such  a  tube,  having  a  glass  railway  on 
which  is  placed  a  roller  with  mica  vanes,  and  the 
electrodes  so  placed  that  the  upper  vanes  are, in  line 
between  them.  When  an  electric  current  passes 
through  the  tube,  these  vanes,  being  at  zero  potential, 
are  attracted  by  the  higher  potential  of  the  positive 
electrode,  producing  a  rotary  movement  of  the  roller 
from  negative  to  positive ;  the  force  being  sufficient  to 
move  it  up  an  incline. 


Fig.  46 — Rotary  Movement  Reversed. 

Fig.  46  shows  a  tube  in  which  a  wheel  with  mica 
vanes  is  so  mounted  that  its  center  is  in  line  between 
the  positive  electrode,  and  the  center  of  the  negative. 
The  negative  electrode  a  b  is  cup-shaped,  and  its  con- 
cave surface  turned  towards  the  positive :  so  that  the 
lines  of  force  may  be  brought  to  a  focus,  and  concen- 
trated on  the  vanes.  And  between  its  center  and  the 
wheel  is  placed  the  mica  screen  c  d. 

A  magnet,  #,  is  suspended  above  the  tube,  between 


ELECTRIC  TRANSMISSION  IN  VACUA.  151 

the  screen  and  negative  electrode,  in  such  a  manner  that 
it  can  be  turned  so  as  to  reverse  the  position  of  its  poles. 

By  this  means  the  electric  current  may  be  attracted 
or  repelled,  so  as  to  pass  over  or  under  the  screen. 
When  it  passes  over  the  screen,  the  upper  vanes  are 
attracted  towards  ihe  positive  electrode,  producing 
rotation  of  the  wheel  in  accord- 
ance with  such  movement :  but 
when  the  position  of  the  magnet 
is  reversed,the  current  is  repelled 
and  passes  under  the  screen,  and 
the  lower  vanes  are  attracted, 
reversing  the  rotation. 

The  glass  in  these  tubes  is 
usually  of  very  low  insulating 
power,  much  lower  than  that 
of  air  at  the  ordinary  density. 
Hence  the  electric  resistance  in 
high  vacua,  being  much  greater 
than  in  the  glass,  electric  move- 
ment takes  place  through  the 
vacua  and  glass  respectively,  in 
the  inverse  ratio  of  the  resist- 
ance of  each. 

Fig.  47  represents  a  tube  in 
which  the  negative  electrode 

.   ,          /.          IIP         i .     i  /.      Fig.  47 — Glass  Illuminated 

consists   of  a   half  cylinder   of 

aluminium,  supported,  near  the  center  of  the  tube,  on  a 
small  glass  tube,  b;  through  which  a  copper  wire  ex- 
tends, connecting  the  aluminium  with  the  platinum 
terminal  below. 

The  ends  of  both  electrodes  come  near  the  walls  of 
the  tube;  and  when  the  electric  charge  passes,  its  prin- 


152  ELEMENTS  OF  STATIC  ELECTRICITY. 

cipal  effect  is  produced  on  the  glass,  which  gives  a  brill- 
iant green  light ;  the  illuminated  surface  terminating  in 
points  near  the  extremities  of  the  negative  electrode. 

The  influence  of  induction  on  the  walls  of  the  tube, 
as  well  as  the  conductivity  of  the  glass,  is  illustrated  in 
Fig.  48 ;  which  represents  a  pear-shaped  tube,  having 
for  its  positive  electrode  an  aluminium  cross,  £>,  placed 
near  its  broad  end;  the  negative  electrode  a  being 
cup-shaped  as  in  Fig.  46.  This  cross  is  hinged  at  bot- 
tom to  the  platinum  terminal;  so  that,  by  a  movement 
of  the  tube,  it  can  easily  be  brought  to  a  horizontal  or 
a  vertical  position. 


Fig.  48— Inductive  Action  of  Metal  Screen. 

When  the  charge  is  passed  through  the  tube,  the 
cross,  when  vertical,  as  shown  in  the  cut,  exerts  a  strong 
inductive  influence  on  the  broad  end  of  the  tube,  to  the 
left;  over  a  space  inclosed  by  lines  extending  over  its 
edges,  from  the  negative  electrode :  repelling  electricity 
from  this  space,  and  screening  it  from  the  action  of  the 
negative  electrode,  which  attracts  electricity  from  the 
other  parts  of  the  tube,  and  from  the  surrounding  air. 


ELECTRIC  TRANSMISSION  IN  VACUA.  153 

Hence  electric  action  within  this  space  is  neutralized  ; 
producing  the  dark  shadow  c  d  shown  on  the  broad 
end ;  while  the  rest  of  the  tube  is  illuminated. 

When  the  screen  is  thrown  down  a  luminous  cross 
takes  the  place  of  the  dark  shadow:  but  this  higher 
illumination  soon  fades,  since  electric  action  on  this 
space  is  now  the  same  as  on  the  rest  of  the  tube. 

If  the  tube  be  used  again,  after  a  period  of  rest,  the 
shadow  can  be  reproduced ;  but  is  never  so  strong  as  at 
first.  This  proves  that  the  glass  has  been  subjected  to 
an  electric  strain,  which  has  permanently  lessened  its 
insulating  power. 

The  illumination  of  the  glass  is  due  to  its  resistance ; 
just  as  the  bright  spark  is  due  to  the  resistance  of  air 
at  the  ordinary  density,  and  the  faint  glow,  to  the 
reduced  resistance  in  vacuum.  Hence,  when  electric 
action  begins,  after  the  screen  is  thrown  down,  the 
resistance  being  greater  on  the  spot  which  was  pro- 
tected by  the  screen,  we  have  the  bright  cross  where 
the  dark  one  was :  but  when  the  electric  strain  has  so 
affected  the  relations  of  the  molecules  to  each  other,  as 
to  lessen  the  resistance,  this  first  bright  glow  ceases,  and 
the  illumination  is  the  same  as  in  the  rest  of  the  tube. 

This  action  on  the  glass,  as  shown  in  Figs.  47  and  48, 
is  accompanied  with  heat  as  well  as  light ;  the-  tube 
shown  in  Fig.  47  becoming  intensely  hot,  at  those 
points  where  the  greatest  electric  energy  is  concen- 
trated. 

Fig.  49  represents  a  tube  constructed  to  show  this 
heating  effect  in  a  very  striking  manner.  Its  upper 
part  is  enlarged  into  a  globular  form :  and,  at  the  bot- 
tom, is  the  concave  negative  electrode,  of  aluminium, 
already  described ;  which  is  so  placed  that  it  brings 


154 


ELEMENTS  OF  STATIC  ELECTRICITY. 


the  lines  of  force  to  a  focus  on  a  piece  of  iridio-platinum, 
£>,  placed  in  the  center  of  the  globe.  This,  being  a 
metal  of  high  resistance,  becomes  white  hot  under  the 
electric  action  ;  glowing  with  intense  brilliancy,  and 
finally  melting. 

The  walls  of  the  globe,  being  remote  from  the  line 

between  the  electrodes, 
which  is  comparatively 
short,  the  glass  is  less  af- 
fected than  in  the  long 
narrow  tubes:  so  that  elec- 
tric action  is  chiefly  con- 
centrated on  the  object  at 
the  center. 

Crookes  attributes  all 
these  phenomena  to  the 
impact  of  the  residual  air 
molecules,  which  he  desig- 
nates as  "radiant  matter"; 
and  claims  that  the  mole- 
cules move  independently 
of  each  other,  and  are 
driven  with  such  force 
against  the  glass  and  other 
objects,  as  to  produce  the 
various  phenomena  de- 
scribed. 

Gordon  considers  this  theory  reasonable,  and  elabo- 
rates it  at  considerable  length  :  but  it  is  not  generally 
accepted  ;  and  it  is  believed  that  the  explanations  here 
given  will  be  found  more  in  accordance  with  well  estab- 
lished electric  principles. 


Fig.  49-Heat  Produced  in  High 
vacua. 


CHAPTER   XL 
ELECTROMETERS. 

PROGRESS  in  every  department  of  science  is  largely 
dependent  on  exact  measurement,  since  it  is  only  by 
this  means  that  we  get  an  accurate  knowledge  of 
relative  values.  The  thermometer  enables  us  to  in- 
vestigate the  laws  of  heat;  the  barometer  gives  us  a 
knowledge  of  atmospheric  pressure,  and  the  various 
matters  relating  to  it.  And  in  chemistry  and  astron- 
omy almost  every  step  depends  on  such  measurement. 
Even  our  ordinary  business  transactions,  and  the  value 
of  our  currency,  are  regulated  by  the  common  scales, 
by  which  we  measure  the  force  of  gravity. 

Electric  science  is  no  exception  to  this  rule.  We 
require  to  know,  accurately,  relative  differences  of 
potential;  the  conductivity  and  resistance  of  various 
substances ;  the  force  of  electric  attraction  and  repul- 
sion, the  comparative  energy  of  the  various  instruments 
used  for  generating  and  accumulating  electricity ;  and 
other  matters  of  similar  importance. 

But  electric  measurement  presents  peculiar  difficul- 
ties not  met  with  in  the  measurement  of  other  forms 
of  energy.  In  the  measurement  of  gravity,  we  deal 
with  a  force  easily  controlled,  the  direction  of  whose 
movement  is  always  known,  and  which,  on  the  various 
parts  of  the  earth's  surface,  is  subject  to  but  slight 
variation. 


156 


ELEMENTS  OF  STATIC  ELECTRICITY. 


In  heat  we  have  a  force,  susceptible  of  easy  control ; 
its  movement  slow,  and  its  direction  easily  ascertained. 
But  electricity  moves  with  the  rapidity  of  thought; 
its  direction  is  difficult  to  ascertain ;  and  it  defies  our 
utmost  efforts  at  absolute  control;  so  that  the  results 
of  measurement,  by  our  best  constructed  instruments, 
fall  short  of  perfect  accuracy. 

In  static  electricity  less  progress  has  been  made  in 
measurement  than  in  other  forms  of  electric  energy, 
whose  practical  applications  are  more  numerous. 

The  electroscope,  sometimes 
classed  with  electrometers,  in- 
dicates the  presence  of  an  elec- 
tric charge,  but  cannot  be  said 
to  measure  it,  except  as  such 
indication  may  show  an  in- 
crease or  diminution  of  a  light 
charge.  Lane's  unit  jar  may 
be  considered  an  electrometer, 
and  the  methods  of  measure- 
ment by  it,  and  by  sparks 
from  the  Holtz  and  Topler 
machines,  belong  to  the  same 
class :  but  both  methods  are 
very  inaccurate,  and  can  be 
used  only  in  special  cases. 

COULOMB'S  TORSION  BALANCE.  —  To  Coulomb  is 
due  the  credit  of  the  first  efforts  at  accuracy  in  elec- 
tric science ;  and  the  torsion  balance,  which  is  still 
extensively  used,  was  his  invention  and  may  properly 
be  regarded  as  the  first  electrometer. 

It  is  represented  by  Fig.  50;  and  consists  of  a  glass 
cylinder  A  J.,  to  the  top  of  which  is  attached,  at  the 


Fig.  50— Coulomb's  Torsion 
Balance. 


ELECTROMETERS.  157 

center,  a  glass  tube  D  D,  to  each  end  of  which  is  fitted 
a  brass  collar.  An  enlarged  section  of  the  upper  end 
of  this  tube  and  its  attachments,  representing  what  is 
known  as  the  torsion  head,  is  shown  separately ;  in 
which  it  will  be  noticed,  that  the  brass  collar  a  has 
fitted  to  it  a  cap  b  with  a  projecting  rim ;  on  the 
upper  surface  of  which  is  a  graduated  scale,  of  360 
equal  divisions.  This  cap  is  capable  of  being  turned 
horizontally,  so  as  to  bring  the  several  divisions  of  the 
scale  under  a  pointer  <?,  attached  to  a. 

In  the  center  of  b  is  a  close  fitting  brass  rod  c?, 
with  a  broad  head  by  which  it  can  be  turned,  when  b 
is  held  firmly ;  or  the  rod  may  be  allowed  to  turn  with 
b.  Attached  to  this  rod  is  a  fine  wire,  which  sustains, 
at  its  lower  extremity,  a  horizontal  shellac  rod  /, 
carrying  at  one  end  a  small  gilt  ball  g.  Opposite  this 
ball,  on  the  cylinder  A  A,  is  &  graduated  scale  a  a, 
having  360  divisions,  to  correspond  to  those  of  the 
upper  scale.  Opposite  the  zero  of  this  scale  is  a  gilt 
ball  gf,  of  the  same  size  as  the  other  gilt  ball,  and  sup- 
ported by  a  shellac  rod  /",  by  which  the  ball  can  be 
introduced  through  an  opening  in  the  top  of  A  A. 

The  instrument  is  supported  on  a  base,  having  level- 
ing screws ;  and  the  air,  in  the  interior,  kept  dry  with 
chloride  of  calcium. 

To  use  this  instrument,  the  cap  b  is  turned  till  the 
zero  of  the  upper  scale  is  brought  under  the  pointer  c. 
The  rod  d  is  then  turned  till  the  movable  ball  g  just 
touches  the  fixed  ball  </,  without  torsion  of  the  wire. 
The  zeros  of  the  two  scales  will  then  be  practically  in 
the  same  vertical  plane. 

The  fixed  ball  cf  is  then  taken  out,  electrified,  and 
replaced  as  before,  in  contact  with  the  movable  ball  g. 


158  ELEMENTS  OF  STATIC  ELECTRICITY. 

Both  being  the  same  size,  the  charge  is  equally  divided 
between  them ;  and,  being  at  the  same  potential,  the 
movable  ball  g  is  repelled  to  a  distance  indicated  by 
the  number  on  the  lower  scale :  at  which  point  the 
force  of  repulsion  is  balanced  by  the  torsion  of  the  wire. 

The  cap  b  is  then  turned  in  opposition  to  the  repul- 
sion, so  as  to  bring  the  ball  g  nearer  to  g'\  the  distance 
being  indicated  on  the  upper  scale.  The  torsion  of  the 
wire  is  thus  increased,  and  repulsion  again  balanced  by 
torsion  in  the  new  position. 

It  is  known  that  the  force  of  torsion  is  proportional 
to  the  angle  of  torsion  :  and  since  this  force  has  to  be 
increased  to  oppose  the  increase  of  repulsion,  as  the 
balls  are  brought  closer,  the  point  to  be  determined  is 
the  ratio  of  increase  of  force,  as  compared  with  the 
reduction  of  distance  between  the  balls;  which  is  done 
by  comparing  the  distances  from  zero,  indicated  on  the 
upper  and  lower  scales. 

The  following  is  one  of  Coulomb's  experiments  for 
this  purpose :  The  first  distance  to  which  the  movable 
ball  g  was  repelled  being  36°,  it  was  found  necessary, 
in  order  to  reduce  this  distance  to  18°,  to  turn  the  cap 
b  through  126°;  and  to  reduce  the  distance  to  8£°  re- 
quired an  additional  rotation  of  the  cap  through  441°. 

The  distances  36°,  18°,  and  8i°  are  to  each  other, 
practically,  in  the  ratio  of  1,  i,  and  I ;  and  the  forces 
of  repulsion  at  these  points  were  balanced  by  torsions 
of  36°,  of  126°+  18°=  144°,  and  of  441° 4  126° -f  8£°= 
5751°,  respectively. 

Now  since  144  =  4x36,  and  575|  (practically  576) 
=16x36,  it  will  be  seen  that  as  the  distance  between 
the  balls  is  divided  by  2  or  by  4,  the  force  of  repulsion 
is  multiplied  by  4  or  by  16 ;  and  thus  Coulomb  proved 


ELECTROMETERS 


159 


that  electric  repulsion  varies  inversely  as  the  square  of  the 
distance. 

INACCURACY  OF  THE  TORSION  BALANCE. — In  the 
use  of  this  instrument,  as  above,  the  arc  is  assumed  as 
the  distance  between  the  balls,  while  the  actual  dis- 
tance is  the  chord  of  the  arc ;  but  since  these  distances 
are  in  the  same  proportion,  the  accuracy  of  the  results 
is  not  affected. 

It  is  also  assumed  that  the  arm  of  the  lever,  by  which 
repulsion  produces  torsion,  is  the  distance  from  the 
center  of  motion  to  the  cen- 
ter of  the  ball  g.  But  this 
is  true  only  when  the  balls 
are  in  contact.  In  every 
other  position,  this  arm  is 
represented  by  a  perpendic- 
ular from  the  center,  on  the 
chord  which  cuts  the  centers 
of  the  two  balls :  and  as  the 
ball  g  moves  round,  and  the 
chord  increases  in  length, 
this  perpendicular  decreases  ; 
and  vanishes  when  the  chord 
equals  the  diameter. 

This  is  shown  in  Fig.  51,  where  b  represents  the  first 
position  of  the  balls,  when  the  arm  equals  a  b :  but 
when  g  moves  round  to  6*,  the  line  a  f  represents  the 
arm ;  and  when  it  moves  to  d,  the  short  line  a  m  rep- 
resents the  arm ;  and  at  n  the  arm  vanishes. 

This  may  be  made  more  plain,  by  considering  that 
the  ball  g  is  moving  under  the  influence  of  two  forces, 
electric  repulsion,  and  the  rigidity  of  the  shellac  rod, 
by  which  it  is  held  at  a  fixed  distance  from  the  center. 


Fig.  51 — Arm  and  Angle  of 
Repulsion  Illustrated. 


160  ELEMENTS  OF  STATIC  ELECTRICITY. 

When  motion  begins,  at  5,  these  forces  act  at  right 
angles  to .  each  other  but  as  the  ball  moves  round,  the 
angle  of  repulsion  constantly  decreases;  being  repre- 
sented at  £,  by  the  angle  a  b  c;  and  at  d,  by  the 
angle  a  b  d;  and  vanishing  at  w,  where  the  two  forces 
are  in  direct  opposition. 

In  this  position  the  force  of  repulsion  opposes  further 
movement :  for,  as  it  radiates  equally  in  every  direction 
from  the  two  balls,  its  force  on  opposite  sides  of  n  is 
equal.  But  since,  in  the  experiment  given,  the  greatest 
angle  was  36°,  which  is  only  one-fifth  of  the  semi-circle, 
the  error  is  not  sufficient  to  affect  the  result  seriously. 

Another  inaccuracy  results  from  lack  of  rigidity  in 
the  fine  wire,  which  causes  it  to  deviate  slightly  from  a 
true  vertical,  under  the  influence  of  repulsion ;  moving 
its  lower  extremity  out  of  the  center. 

There  is  also  a  slight  inaccuracy  resulting  from  the 
force  of  repulsion  being  estimated  from  the  centers  of 
the  balls,  instead  of  from  their  nearest  points. 

It  is  also  assumed  that  electric  repulsion  remains 
constant  during  the  experiment:  which  would  not  be 
strictly  true;  since  there  is  a  continual  reduction  of 
energy,  from  causes  already  explained,  which  would 
produce  serious  error,  if  the  experiment  were  of  long 
duration. 

Since  each  ball  becomes  a  center  of  electric  radiation, 
it  is  evident  that  the  lines  of  force  cut  by  each  repre- 
sent but  a  very  small  part  of  the  entire  repulsive  energy. 
But  since  the  balls  are  of  equal  size  and  equal  poten- 
tial, it  may  be  assumed  that  the  proportion  between 
the  energy  actually  measured,  and  the  entire  energy,  is 
the  same  in  each  ball.  But  an  instrument  embracing 
all  the  lines  of  force  would  evidently  be  more  reliable. 


ELECTROMETERS.  161 


These  inaccuracies  doubtless  account  fWcJhe  slight 
error  observed  in  Coulomb's  experiment,  am^ifrnd  to 
confirm  the  correctness  of  his  results  by  showing  suf- 
ficient cause  for  the  error. 

ATTRACTED-DlSC     ELECTROMETERS. — Sir    W.    SnOW 

Harris  was  the  first  to  construct  an  electrometer  on  the 
attracted-disc  principle.  His  instrument  consisted  of  a 
scale  beam,  carrying  at  one  end  a  pan  for  the  weights, 
balanced  at  the  other  end  by  an  insulated  metal  disc,  sus- 
pended horizontally  over  a  similar  fixed,  insulated  disc. 

An  electric  charge  being  given  to  the  lower  disc,  the 
force  of  attraction  between  it  and  the  upper  disc  was 
measured  by  weights  placed  in  the  scale  pan. 

The  rapid  loss  of  charge,  from  the  edge  of  the  elec- 
trified disc,  was  the  chief  objection  to  this  instrument. 
But  the  principle  has  been  adopted,  and  the  construc- 
tion improved  by  Sir  William  Thomson,  whose  instru- 
ment, shown  by  Fig.  52,  is  described  as  follows: — 

THOMSON'S  ABSOLUTE  ELECTROMETER. — This  in- 
strument consists  of  two  distinct  parts;  one  for  testing 
and  maintaining  a  certain  constant  auxiliary  potential 
V,  and  the  other  for  determining,  in  absolute  measure, 
the  difference  between  the  potentials  of  any  two  given 
conductors.  The  first  of  these  parts  embraced  a  Ley- 
den  jar,  forming  the  case  of  the  instrument,  an  idio- 
static  gauge,  -and  a  replenisher  E. 

The  Leyden  jar  is  a  glass  cylinder,  closed  at  top  and 
bottom  by  metal  plates;  and  coated,  inside  and  out, 
with  tin-foil,  in  which  openings  are  left  for  viewing  the 
internal  parts;  and  an  uncoated  surface,  for  insulation, 
left  at  the  top  and  bottom,  between  the  inner  coating 
and  the  metal  plates. 

The  idiostatic  gauge  will  be  understood  from   Fig. 


162  ELEMENTS  OF  STATIC  ELECTRICITY. 


Fig.  52— Thomson's  Absolute  Electrometer. 


ELECTROMETERS.  163 

53.  A  small  aluminium  plate  P  is  fitted  to  a  square 
hole  in  the  metal  plate  6r,  like  a  trap-door,  without 
touching  the  edges.  To  one  side  of  P  is  attached  an 
arm  A,  of  the  same  material,  enlarged  at  its  junction 
with  P,  and  bent,  so  that  when  the  surfaces  of  P  and 
Gr  are  in  the  same  plane,  the  arm  is  elevated  a  little 
above  6r,  and  is  parallel  with  it. 

A  platinum  wire  /  stretched  between  two  supports, 
attached  to  (7,  passes  through  the  enlarged  part  of  the 
arm  ^,  over  a  slight  projection ;  supporting  P,  and,  by 
its  torsion,  regulating  its  movements.  At  the  outer 
end  of  the  arm  is  a  fork 
F ;  and  between  its 
prongs  is  a  little  white 
enameled  standard,  at- 
tached to  G- ;  having, 
on  its  outer  face,  two 
black  dots,  close  to- 

gether,  and  in  the  same  Fig  53_The  Idiostatic^T 
vertical  line.  A  black 
hair,  stretched  across  the  fork,  and  viewed  through  the 
lens  Z,  moves  up  and  down  in  front  of  the  dots ;  and 
conies  exactly  between  them,  when  the  surfaces  of  P 
and  G-  are  in  the  same  plane.  This  is  called  the  sighted 
position. 

Under  the  plate  G-  is  seen,  in  Fig.  52,  a  circular 
metal  plate  F,  supported  on  a  metal  rod,  attached  to 
the  metal  plate  A,  which  is  in  contact  with  the  inner 
coating  of  the  Ley  den  jar;  so  that  A  and  Pare  always 
at  the  same  potential  V,  as  this  coating.  The  distance 
between  F  and  6r  is  so  regulated,  that  when  the  poten- 
tial of  F  is  V,  its  attraction  for  the  plate  P  overcomes 
the  torsion  of  the  platinum  wire,  and  keeps  P  in  the 


164 


ELEMENTS  OF  STATIC  ELECTRICITY. 


sighted  position :  and,  in  this  way,  the  constancy  of  the 
potential  J^is  tested. 

This  constancy  of  potential  is  maintained  by  the 
replenisher  seen  at  JS,  which  is  practically  a  small  To- 
pler  machine ;  and  is  shown  separately  in  Fig.  54.  A 
and  B  are  two  insulated  metal  inductors,  to  which  are 
attached  two  receiver  springs  a  and  b.  C  and  D  are 
two  contact  springs,  in  electric  connection  with  each 
other,  but  insulated  from  the  other  parts. 

P  and  Q  are  two  metal 
carriers,  attached  to  an  eb- 
onite cross-piece,  through 
which  passes  the  ebonite 
axis  T,  which  can  be  ro- 
tated by  the  milled  head 
E:  so  that  the  carriers 
P  and  $,  revolving  inside 
the  inductors  A  and  B, 
shall  successively  touch 
the  springs  a,  J>,  b,  0. 

When  the  replenisher 
is  in  its  place,  as  shown 

in  Fig.  52,  the  inductor  A  is  put  in  electric  connection 
with  the  disc  A  ;  which  is  supported  in  connection  with 
the  inner  coating  of  the  Leyden  jar :  while  the  inductor 
B,  being  in  contact  with  the  cover,  is  in  electric  con- 
nection with  the  outer  coating.  And  since  the  replen- 
isher operates  on  the  principle  of  the  Topler  machine, 
already  described,  its  rotation,  either  direct  or  reversed, 
will  raise  or  lower  the  potential  of  the  jar :  and  so  keep 
the  potential  of  the  plate  A,  and  of  the  idiostatic  gauge, 
connected  with  it,  at  the  constant  potential  F",  as 
shown  by  the  gauge. 


Fig.  54— The  Replenisher. 


ELECTROMETERS.  m  165 

The  second  part  of  the  electrometer  consists  of  the 
apparatus  for  expressing  differences  of  potential,  be- 
tween conductors,  in  absolute  measure.  The  metal 
plate  A,  called  the  guard  plate,  has,  at  its  center,  a 
circular  opening  about  If  inches  in  diameter,  to  which 
is  fitted  the  disc  (7;  which  just  fills  it  without  touching 
the  edges;  and  is  made  of  thin  aluminium,  flat  and 
smooth  on  its  under  side,  but  strengthened  by  a  rim, 
and  radial  arms,  on  its  upper  side.  It  is  supported  by 
three  light  steel  springs,  shaped  somewhat  like  tuning- 
forks,  and  placed  horizontally,  at  equal  distances  apart ; 
one  of  which  is  shown  at  S.  The  lower  end  of  each  is 
attached  to  the  center  of  (7,  and  the  upper  end  to  a 
brass  socket,  which  is  cemented  to  the  lower  end  of  a 
glass  rod,  shown  at  /;  which  insulates  it  from  the 
metal  rod  above ;  to  the  lower  end  of  which  the  glass 
rod  is  attached.  And  the  metal  rod  is  moved  vertically 
in  guides  by  the  micrometer  screw  M ;  the  movements 
being  registered  by  the  scale  6r,  and  the  graduated  disc 
D. 

To  the  center  of  the  disc  0  is  attached  a  fine  hair: 
in  front  of  which  a  lens,  II,  is  so  placed  as  to  form,  at 
its  conjugate  focus,  near  the  surface  of  the  jar,  an 
image  of  the  hair ;  which  may  be  viewed  through  the 
eye-piece  at  L.  This  image  is  seen  exactly  between 
the  points  of  two  screws  K,  when  the  lower  surfaces 
of  the  disc  (7,  and  guard  plate  A,  are  in  the  same  plane  : 
which  is  called  the  sighted  position. 

On  a  support  below  A,  is  the  metal  disc  B,  known 
as  the  attracting  disc  ;  insulated  from  the  jar,  and  mov- 
able vertically  by  the  micrometer  screw  M' ;  the  move- 
ments being  registered  by  the  scale  R,  arid  the  grad- 
uated disc  T.  It  is  connected  with  the  electrode  -ZV",  by 


166  ELEMENTS  OF  STATIC  ELECTRICITY. 

which  it  can  be  put  in  electric  connection  with  bodies 
whose  potential  is  to  be  tested. 

The  attracted  plates  P  and  C  are  really  movable 
centers  of  the  guard  plates  Cr  and  A;  and  since  loss  of 
charge,  from  radiation  and  otherwise,  affects  chiefly  the 
outer  edges,  the  small  centers  are  practically  unaffected 
by  such  loss.  Hence  the  large  discs  Cr  and  A  are 
appropriately  called  guard  plates. 

MODE  OF  USING  THE  ABSOLUTE  ELECTEOMETEE. — 
The  plates  are  first  brought  to  zero  potential,  by  put- 
ting them,  for  an  instant,  in  electric  connection,  by  the 
electrode  -ZV,  connecting  with  Z?,  and  a  wire  connecting 
with  A  through  the  cover.  The  disc  O  is  then  brought  to 
its  sighted  position  by  the  micrometer  Jf,  and  the  read- 
ing noted.  A  known  weight,  w,  is  then  placed  upon  it 
so  as  to  depress  it  below  the  level  of  the  guard  plate 
A;  and  M  is  turned  till  C  is  again  raised  to  its  sighted 
position :  the  reading  is  noted,  and  the  weight  removed. 

The  Leyden  jar  is  then  charged  to  potential  V,  as 
determined  by  the  idiostatic  gauge,  and  kept  constant 
by  the  replenisher,  during  the  experiment.  The  disc 
B  is  now  put  into  connection  with  the  outside  coating 
by  the  electrode  N;  and  the  micrometer  Mr  turned  till 
the  attraction  of  B  on  the  disc  C  brings  it  again  to  its 
sighted  position.  Hence  the  attraction  of  B  is  known  to 
be  equal  to  the  weight  w.  This  reading  being  noted,  B 
is  insulated,  and  the  bodies,  the  difference  of  whose  po- 
tentials x  and  z  is  required,  are  successively  put  into 
contact  with  B  through  N.  The  distances  d  and  h 
through  which  B  has  to  be  moved  to  bring  the  disc  (7, 
in  each  case,  to  its  sighted  position,  are  noted,  and  the 
difference  of  potential  can  then  be  calculated. 

FOE      CHAEGED     SUEFACES.  —  With     a 


ELECTROMETERS.  167 

given  charge,  the  electric  energy  at  any  point  on  a  con- 
ductor, called  its  surface  density,  is  in  proportion  to  its 
surface  area.  Let  Q  represent  the  surface  density,  then 
the  electric  force,  exerted  by  a  charged  conductor  on  a 
point  near  it,  equals  Q  multiplied  by  the  surface  area. 

On  a  sphere  the  surface  equals  the  square  of  its  ra- 
dius multiplied  by  4x3.14159.  If  3.14159  =  ^,  and 
radius  =  1,  we  have  I2  x  4  a  =  4  n.  Hence  the  force 
exerted  by  a  charged  sphere  on  a  point  near  it  equals 
4  7t  Q  ;  and  the  force  exerted  by  a  charged  hemispher- 
ical surface  equals  2?tQ. 

The  hemispherical  surface  may  be  considered  as 
made  up  of  the  bases  of  an  infinite  number  of  small 
cones,  having  their  apexes  at  the  center.  Hence  each 
base  subtends  a  solid  angle  :  and  lines  of  force,  extend- 
ing from  surface  to  center,  are  everywhere  normal  to 
the  surface. 

Now  if  we  conceive  a  plane  surface  applied  to  the 
hemispherical  surface,  and  these  cones  extended  to 
meet  it ;  we  find  that  the  lines  of  force,  extending  from 
these  bases,  are  oblique  to  the  plane  surface.  Hence 
each  one  can  be  resolved  into  two  components,  one 
normal  to  the  plane,  and  the  other  acting  along  it. 
But  since  there  are  an  infinite  number  of  these  cones, 
the  lines  of  force  from  whose  bases  may  all  be  resolved 
in  this  way;  the  components  along  the  plane,  all 
around,  neutralize  each  other,  leaving  only  the  normal 
components;  whose  force  equals  the  sum  of  all  the 
solid  angles  multiplied  by  the  surface  density,  which,  as 
we  have  seen,  equals  2  TIQ.  Hence  the  expression  is 
the  same  for  a  plane  or  a  hemispherical  surface. 

APPLICATION  OF  FORMULAE  TO  MEASUREMENTS 
BY  ELECTROMETER. — When  there  are  two  discs,  at 


168  ELEMENTS  OF  STATIC  ELECTRICITY. 

different  potentials,  near  each  other,  as  A  and  B  in  the 
electrometer,  the  attraction  of  each  for  the  other  is 
equal  ;  the  air  being  the  dielectric  between  them. 
Hence  the  force,  exerted  at  any  point  between  them, 
equals  the  force  on  both  surfaces,  represented  b}T  ^TIQ; 
and  tends  to  draw  the  movable  disc  C  towards  B. 
But  this  force  is  also  equal  to  the  difference  of  poten- 
tial, divided  by  the  distance  between  the  discs.  Hence 
when  x  represents  difference  of  potential,  and  d  the 

or 
distance,  the  resultant  force,  at  any  point,  equals   —  . 

x  x 

Hence  4  ?*(>=—  ,  and  Q  = 
a 


Now  if  the  surface  of  the  movable  disc  O  be  repre- 
sented by  s,  its  attractive  force  will  equal  s  Q  :  hence 
the  total  attractive  force  equals  2  TCQ  x  s  Q  =  2  it  s  if. 

nr 

And   substituting   for   Q   its   value,    -7  —  —  ,    we    have 

4t  Tt  Ct 

/     x     V      0  x2  sx2 

2^M    A  -  -j)=^nS   1X       2^2=Q  -  ^2' 

\4  it  d'  16  a*  d2      Sad2 

Now  since  the  attractive  force  equals  the  weight  w, 
multiplied  by  the  acceleration  produced  by  gravity, 

s  x2 

represented  by  #,  we  have  w  g  =  -   —  :    therefore  x 

" 


n 


=  d  —  ^  (1),  which  expresses  x  in  absolute  meas- 

ure.    But  x  represents  the  potential  of  the  first  body 
tested  by  the  electrometer. 

By  a  similar  process  the  potential,  2,  of  the  second 

body  is  expressed  by  the  equation,  z  =  h   I—      ^  (2). 

Subtracting  (2)  from  (1),  we  have  x  —  z  =  (d  —  Ji) 
n  w  g 


J 


ELECTROMETERS.  169 

By  substituting  figures  for  the  letters  in  the  second 
member  of  this  equation,  the  difference  of  potential,  of 
any  two  bodies  we  wish  to  test,  may  be  expressed 
arithmetically.  ,— 

I    O  7t  W  C/ 

The  expression  J ^-  is  constant ;  since  it  rep- 
resents the  attraction  of  the  disc  B  for  (7,  when  the 
Ley  den  jar  is  at  the  constant  potential,  V:  while  the 
expression  (d  —  A)  is  variable;  representing  the  differ- 
ence of  distance,  required  by  the  variable  difference  of 
potential,  expressed  by  x — z. 

THOMSON'S  QUADRANT  ELECTROMETER. — This  in- 
strument, invented  by  Sir  William  Thomson,  is  highly 
esteemed  for  its  great  sensitiveness.  It  is  represented 
by  Fig.  55,  and  consists  of  a  frame  supporting  a  Leyden 
jar,  which  resembles  an  inverted  glass  shade,  with  a 
brass  cover,  to  which  the  principal  parts  are.  attached. 

These  consist  of  the  idiostatic  gauge  and  replenisher, 
already  described,  and  the  quadrants  and  needle,  and 
parts  connected  with  them. 

The  jar  contains  strong  sulphuric  acid :  which  forms 
the  inner  coating,  keeps  the  interior  free  from  moist- 
ure, and  forms  a  perfect  connection  with  the  needle, 
without  friction.  The  outer  coating  consists  of  strips 
of  tin-foil,  connected  with  the  cover  and  supporting 
frame.  The  upper  part  of  the  jar  incloses  the  quad- 
rants and  needle ;  protecting  the  needle  from  currents 
of  air,  and  permitting  its  movements  to  be  seen. 

Fig.  56  is  an  enlarged  view  of  the  needle  and  quad- 
rants. The  needle  is  a  thin,  flat  piece  of  aluminium, 
shaped  like  a  figure  8 ;  represented  by  the  dotted  lines 
in  Fig.  56  ;  and  seen  edgewise  in  its  place  at  w,  in  Fig. 
55.  Through  its  center  passes  a  piece  of  stout  platinum 
wire  to  which  it  is  attached,  and  which  terminates 


170  ELEMENTS  OF  STATIC  ELECTRICITY. 

above  in  a  small,  T-  shaped  piece  of  metal :  to  which 
are  attached,  at  the  extremities  of  the  cross  piece,  two 
fibers  of  unspun  silk ;  by  which  the  needle  is  suspended 


Fig.  55 — Thomson's  Quadrant  Electrometer. 

from  a  projecting  arm,  supported,  in  the  upper  part  of 
the  instrument,  on  a  vertical  glass  rod.  When  the 
needle  is  at  rest,  in  the  fixed  position  between  the 
quadrants,  as  shown  in  Fig.  56,  the  silk  fibers  hang 
parallel  to  each  other,  and  the  cross  piece,  below,  is 


ELECTROMETERS. 


171 


then  parallel  to  the  projecting  arm  above.  But  in  any 
other  position,  each  fiber  is  at  an  angle  with  its  vertical 
position,  and  the  needle  slightly  elevated :  conse- 
quently the  force  of  gravity  tends  constantly  to  turn 
the  needle,  without  friction,  back  to  its  fixed  position. 
This  mode  of  suspension  is  termed  bifilar. 

A  platinum  weight,  suspended  in  the  sulphuric  acid 
by  a  fine  platinum  wire,  from  the  lower  end  of  the  stiff 
wire  below  the  needle,  keeps 
the  needle  in  position,  and  in 
contact  with  the  inner  coat- 
ing. 

The  wire,  above  and  below 
the  needle,  is  inclosed  in  fixed 
guard  tubes ;  the  lower  one 
shown  at  w :  which  screen  it 
from  external  electric  influ- 
ence; and  furnish  a  COimeC-  Fig.  56-Quadrants  and  Needle. 

tion,  by  which  the  charge  is  given  to  the  inner  coating. 

The  needle  is  inclosed  within  four  brass  quadrants  : 
which,  if  joined,  would  form  a  circular  box.  They  are 
separated  from  each  other,  and  from  the  needle,  as 
shown  in  Fig.  56 :  and  opposite  pairs,  A  and  A',  B  and 
Bf,  are  connected  by  fine  wires ;  and  all  supported  at 
the  same  level ;  and  insulated,  by  glass  rods  attached 
to  the  cover. 

Three  of  them  are  permanently  attached,  but  the 
fourth  can  be  moved  in  and  out  horizontally ;  guides, 
and  a  spring  and  counteracting  screw,  being  arranged 
to  keep  it  in  position,  and  regulate  its  movement. 

Above  the  needle,  and  attached  to  its  supporting 
wire,  is  a  small  concave  mirror  t;  by  which  a  ray  of 
light  is  reflected  on  a  scale,  placed  in  front  of  it,  at  a 


172  ELEMENTS  OF  STATIC  ELECTRICITY. 

distance  of  about  36  inches.  This  scale  is  shown  in 
Fig.  57.  Behind  it  is  a  lamp,  the  light  from  which 
comes  through  a  vertical  slit  in  a  screen  :  above  which 
is  a  horizontal  screen,  which  cuts  off  the  direct  rays 
from  below ;  while  the  angle  of  reflection  brings  the 
ray  from  the  mirror  directly  on  the  scale,  where  it 
appears  as  a  small  spot  of  light.  Another  screen, 
placed  at  an  angle, 
cuts  off  the  direct 
rays  from  above. 

As  the  mirror  turns 
with  the  needle,  the 
reflected  ray  becomes 
a  long  pointer ;  mov- 
ing without  friction  : 
by  which  the  slight- 
est movement  of  the 
needle  is  indicated  on 

Fig.  57— Scale,  Lamp,  and  Screen. 
In  the  instruments 

first  constructed,  the  needle  was  suspended  by  a  single 
fiber  of  silk ;  and  a  small  magnet  attached  to  the  back 
of  the  mirror :  which,  by  the  attraction  between  it  and 
a  large  magnet,  placed  outside  the  jar,  as  shown  in  Fig. 
55,  controlled  and  limited  the  movements  of  the  needle  ; 
the  attraction  of  the  magnets  tending  constantly  to 
bring  it  back  to  its  fixed  position,  where  the  spot  of 
light  rests  on  the  zero  of  the  scale.  But  the  bifilar  sus- 
pension is  now  preferred ;  rendering  the  use  of  magnets 
unnecessary. 

At  I  and  m.  Fig.  55,  are  seen  the  chief  electrodes ; 
used  to  connect  the  opposite  pairs  of  quadrants  with 
bodies  whose  potential  is  to  be  tested :  and  at  p  is  the 


ELECTROMETERS.  173 

charging  electrode,  used  to  connect  the  replenisher  with 
the  inner  coating  of  the  Ley  den  jar.  One  pair  of  quad- 
rants, A  Af,  Fig.  56,  is  connected  with  the  electrode  ?, 
and  the  other  pair,  B  B',  with  the  electrode  m. 

MODE  OF  USING  THE  QUADRANT  ELECTROMETER. 
— The  Leyden  jar  is  connected  with  the  replenisher  by 
the  electrode  />,  and  charged  to  a  certain  constant  po- 
tential, F",  as  indicated  by  the  gauge ;  and  its  constancy 
maintained  during  the  experiment:  and  the  needle, 
being  connected  with  its  inner  coating,  has  therefore 
the  same  constant  potential  V. 

By  means  of  the  electrodes  I  and  m,  a  connection  is 
then  made  between  the  opposite  pairs  of  quadrants,  and 
any  two  bodies  \vhose  difference  of  potential  is  required ; 
one  of  which  is  usually  the  earth.  Suppose  the  earth 
connection  to  be  made  with  the  electrode  m;  then,  if 
the  potential  of  the  other  body  is  higher  than  that  of 
the  earth,  the  needle  will  move  round  from  the  higher 
to  the  lower  potential ;  that  is,  from  A  A'  to  B  Bf:  but 
if  it  is  lower,  the  movement  will  be  from. B  Bf  to  A  Af: 
and  the  difference  of  potential  will  be  indicated  on  the 
scale  by  the  movement  of  the  spot  of  light,  to  the  right 
or  left  from  zero ;  and  may  be  considered  practically 
correct,  within  certain  limits.  In  this  way  the  re- 
quired potentials  are  compared  with  the  constant  poten- 
tial V;  and  the  results  determined  in  absolute  measure. 

In  the  Helmholtz  quadrant  electrometer  the  quad- 
rants are  maintained  at  the  constant  potential ;  and  the 
bodies  whose  potential  is  required  are  connected  with 
the  needle. 

There  are  various  styles  of  Thomson's  electrometers : 
both  of  the  attracted-disc  and  quadrant  instruments. 
Some  of  them  are  portable,  and  much  simpler  than 


174  ELEMENTS  OF  STATIC  ELECTRICITY. 

those  already  described;  the  replenisher,  gauge,  and 
Leyden  jar,  being  omitted;  also  the  bifilar  attachment 
in  the  quadrant  instrument;  the  movements  of  the 
needle  being  controlled  by  the  torsion  of  a  fine  wire. 
And,  in  the  attracted-disc  electrometer,  the  position  of 
the  discs  is  sometimes  reversed;  the  attracting  disc 
being  placed  above,  in  the  portable  style. 


CHAPTER   XII. 
THE  ELECTRICITY  OF  THE  EARTH  AND  ATMOSPHERE. 


POTENTIAL  AND  EARTH  CURRENTS. 

TERRESTRIAL  and  atmospheric  electricity  are  so  in- 
timately related,  that  to  obtain  a  correct  knowledge  of 
either  requires  the  consideration  of  its  relations  to  the 
other. 

Viewing  electricity  as  a  universal  property  of  mat- 
ter, its  existence  in  the  earth  and  atmosphere  follows 
as  a  necessary  consequence.  Hence,  we  are  to  study 
its  phenomena  in  this  connection,  rather  than  to  ac- 
count for  its  origin.  These  phenomena  pertain  chiefly 
to  difference  of  potential  between  different  parts  of  the 
earth's  surface;  different  parts  of  the  atmosphere;  and 
between  the  earth's  surface  and  the  atmosphere. 

This  \lifference  of  potential  results  from  various 
causes.  We  have  already  seen  how  difference  of  po- 
tential may  be  produced,  artificially,  by  various  instru- 
ments, which  are  combinations  of  different  substances, 
having  different  degrees  of  electric  resistance  and  con- 
ductivity. By  similar  methods  nature,  on  a  grand 
scale,  produces  results  of  which  ours  are  but  feeble  im- 
itations. 

ILLUSTRATIONS  FROM  THE  THERMOPILE. —  In  the 
thermopile  we  have  an  illustration  of  the  method  by 
which  difference  of  potential  is  produced  by  heat.  This 


176  ELEMENTS  OF  STATIC  ELECTRICITY. 

instrument  is  a  combination  of  metal  bars,  whose  con- 
ductivity for  heat  and  electricity  varies  greatly.  A 
number  of  these  bars,  arranged  in  compact  form,  and 
properly  insulated,  are  soldered  together  in  an  alter- 
nating series:  so  that  a  current  of  electricity,  passing 
through  them,  has  to  pass  from  one  metal  to  the  other. 
They  are  folded  together,  and  mounted  in  such  a  man- 
ner, that  heat  may  be  applied  to  one  set  of  junctions; 
while  the  opposite,  alternate  set,  is  cooled. 

In  this  way,  instruments  are  constructed,  in  which  a 
very  slight  difference  of  temperature,  between  the  op- 
posite sets  of  junctions,  creates  a  perceptible  difference 
of  electric  potential:  and  powerful  batteries  are  con- 
structed in  the  same  manner. 

The  earth  may  be  regarded  as  an  immense  battery  of 
this  kind;  being  composed  of  heterogeneous  materials, 
whose  conductivity  for  heat  and  electricity  varies 
greatly:  and  which  are  "subjected  to  great  extremes  of 
temperature,  at  opposite  junctions,  fulfilling  exactly 
the  conditions  of  the  thermopile. 

The  ocean,  a  vast,  homogeneous  conductor,  is  sepa- 
rated into  different  parts  by  the  great  continents;  whose 
conductivity  differs  from  it  greatly:  the  five  great 
divisions  of  the  ocean,  and  the  two  continents,  con- 
stituting an  alternating  series  of  conductors,  of  differ- 
ent conductivities. 

The  surface  of  the  continents,  composed  of  rock  and 
soil,  of  lakes,  rivers,  and  sandy  deserts,  presents  a  great 
diversity  of  material,  of  widely  different  conductivity. 

In  the  torrid  and  frigid  zones,  we  have  the  opposite 
extremes  of  temperature;  which,  in  the  thermo-electric 
battery,  are  produced  by  exposing  one  set  of  junctions 
to  the  heat  of  a  lamp  furnace  ;  while  the  opposite  set  is 


POTENTIAL  AND  EARTH  CURRENTS.  177 

cooled  with  ice.  Similarly  also  in  the  diurnal  revolu- 
tion of  the  earth,  opposite  sides  are  subjected  daily  to  a 
constantly  changing  temperature.  And,  in  its  annual 
revolution,  we  have  the  same  result  in  the  changing  sea- 
sons; which  also  produce  great  changes  in  the  conduct- 
ing character  of  the  surface ;  from  the  frozen,  snow- 
clad  surface  of  winter,  to  the  verdure-clad  surface  of 
summer. 

DLURNAL  AND  SEASONAL  VARIATION. — The  change 
of  electric  potential  produced  by  these  causes  in  the 
earth,  induces  the  opposite  potential  in  the  atmosphere; 
which,  by  its  lower  strata,  is  insulated  from  it.  Hence, 
in  observations  made  on  the  potential  of  the  earth  and 
atmosphere,  we  find,  as  we  should  be  led  to  expect, 
daily  maxima  and  minima  potential,  and  also  seasonal 
maxima  and  minima. 

In  several  series  of  observations,  made  by  different 
observers  in  Europe,  both  on  the  continent  and  in  the 
British  Isles,  these  maxima  and  minima  were  carefully 
noted:  and  it  was  found,  that,  in  winter,  the  daily 
maxima  occur  at  about  10  A.  M.  and  7  P.  M.;  in  sum- 
mer at  about  8  A.  M.  and  10  P.  M.;  and  in  spring  and 
autumn,  at  about  9  A.  M.  and  9  P.  M.  The  daily  min- 
ima occur,  in  summer,  at  about  3  P.M.,  and  midnight; 
but  the  daily  winter  minima  are  not  given  with  suf- 
ficient definiteness  to  be  reliable. 

From  this  we  see,  that  the  daily  maxima,  occurring 
soon  after  sunrise  and  sunset,  correspond  to  the  hours 
of  greatest  change  of  temperature ;  while  the  daily 
minima  occur  at  the  hours  when  temperature  is  most 
constant. 

The  seasonal  maximum  occurs  in  winter,  and  the 
seasonal  minimum  in  summer:  the  maximum  about 

12 


178  ELEMENTS  OF  STATIC  ELECTRICITY. 

January,  and  the  minimum  in  May  and  June.  They 
are  doubtless  due,  in  part,  to  the  different  conduct- 
ivity of  the  earth's  surface  in  summer  and  winter, 
as  already  mentioned ;  and  also  to  the  dry  winter 
atmosphere,  when  atmospheric  insulation  is  high,  as 
compared  with  the  damp  atmosphere  of  spring  and 
early  summer,  when  it  is  low ;  the  greatest  minimum 
occurring  in  the  months  when  our  atmosphere,  in  the 
north  temperate  zone,  is  most  heavily  laden  with  vapor. 
At  this  season  the  earth  is  covered  with  green,  suc- 
culent herbage ;  wet  with  frequent  showers,  and  laden 
at  night  with  heavy  dews ;  forming  a  conducting  sur- 
face, which  offers  but  slight  resistance  to  electric  trans- 
mission. 

Towards  the  close  of  summer,  the  grain  ripens,  the 
showers  become  less  frequent,  the  dews  lighter,  and  a 
vast  expanse  of  dry  straw  and  stubble,  with  a  parched 
soil  beneath  it,  offering  high  electric  resistance,  takes 
the  place  of  the  former  conducting  surface.  As  fall  ad- 
vances, and  the  grass  becomes  dry  and  withered,  and 
the  trees  shed  their  leaves,  there  is  a  constant  increase 
of  this  surface  resistance,  and  a  corresponding  increase 
of  electric  potential,  till  the  winter  maximum  is  reached. 

While  this  difference  of  conductivity  in  the  land 
surface  is  taking  place,  the  conductivity  of  the  water 
surface  remains  practically  constant :  hence  the  period 
of  minimum  potential  corresponds  to  that  in  which  the 
difference  of  conductivity,  between  the  land  and  water 
surfaces,  is  least;  while  the  period  of  maximum  poten- 
tial corresponds  to  that  in  which  it  is  greatest ;  point- 
ing clearly  to  this  difference  as  a  probable  cause. 

In  addition  to  the  changes  of  electric  potential,  in- 
duced in  the  atmosphere  by  these  changes  in  the  elec- 


POTENTIAL  AND  EARTH  CURRENTS.  179 

trie  condition  of  the  earth's  surface,  its  electricity  is 
doubtless  affected,  directly,  by  conditions  similar  to 
those  which  affect  the  earth's  electricity. 

In  its  combination  of  dry  air  arid  watery  vapor ;  the 
one,  an  insulator,  and  the  other,  a  conductor ;  separate 
parts  heated  and  cooled  alternately,  twice  in  twerrty- 
four  hours,  we  have  thermo-electric  conditions  similar  to 
those  already  noticed  in  the  earth's  surface ;  though 
the  resulting  electric  disturbance  is,  perhaps,  less  in- 
tense, as  the  composition  of  the  atmosphere  is  nearly 
uniform,  while  that  of  the  earth's  surface  presents  great 
diversity. 

DIFFERENCE  OF  POTENTIAL  BETWEEN  ATMOS- 
PHERIC STRATA. — Another  cause  of  atmospheric  elec- 
tric disturbance  is  found  in  the  great  difference  of 
electric  resistance  between  the  upper  and  lower  atmos- 
pheric strata;  caused  by  the  density  below  and  rarity 
above.  This  resistance  makes  the  dense  lower  stratum, 
where  most  of  our  observations  are  made,  an  excellent 
insulator ;  while  the  rarity  of  higher  strata  facilitates 
electric  transmission ;  a  constant  decrease  of  resistance 
taking  place,  from  the  lower  to  the  higher,  till  a  point 
is  reached,  where  it  is  reduced  to  that  of  the  ordinary 
Geissler  tube ;  while,  in  still  higher  strata,  the  resist- 
ance increases,  on  account  of  the  extreme  rarity  of  the 
air :  which  equals,  and  finally  exceeds,  that  of  the  best 
vacuum  tubes. 

The  existence  of  a  corresponding  difference  of  elec- 
tric potential  has  been  proved  by  numerous  experi- 
ments ;  among  which  may  be  noted  the  following : — 

From  an  elevated  position,  a  metal  -  pointed  arrow 
was  shot  upward  to  a  vertical  height  of  250  feet:  a  con- 
ducting cord,  connected  with  it,  and  properly  insulated, 


180  ELEMENTS  OF  STATIC  ELECTRICITY. 

communicated  with  an  electroscope  at  its  lower  extrem- 
ity. As  the  arrow  rose,  the  electroscope  showed  a 
steadily  increasing  difference  of  potential,  till  the 
indications  equaled  the  full  capacity  of  the  instrument. 

The  arrow  was  then  shot  horizontally,  at  an  eleva- 
tion of  about  three  feet,  but  no  change  of  potential  was 
indicated ;  proving  that  the  indications  resulted  from  a 
difference  of  potential  existing  in  the  atmosphere,  and 
were  not  due  to  the  friction  of  the  arrow  in  passing 
through  the  air. 

The  difference  of  potential,  in  this  experiment,  was 
between  the  earth  and  atmosphere :  but  the  following 
experiment  was  entirely  independent  of  the  earth. 
During  a  balloon  ascent,  a  conductor,  170  feet  in  length, 
was  lowered  into  the  air ;  a  ball  being  attached  to  its 
lower  end,  and  its  upper  end  connected  with  an  elec- 
troscope. The  indications  showed  a  marked  difference 
of  potential  between  the  upper  and  lower  strata. 

As  the  balloon  moved  with  the  wind,  the  friction 
between  the  ball  and  the  air  could  not  have  been  suf- 
ficient to  affect  the  electroscope  perceptibly ;  so  that, 
in  this  instance,  as  in  the  former,  the  indications  of  the 
instrument  must  be  attributed  to  a  difference  of  po- 
tential existing  in  the  atmosphere. 

The  series  of  observations  already  referred  to,  and 
numerous  others  of  a  similar  character,  prove  that  the 
potential  of  the  atmosphere  is  almost  invariably  positive 
with  reference  to  that  of  the  earth. 

THE  ATMOSPHERE  AS  A  LEYDEN  JAE. — It  is  evi- 
dent that  we  have,  in  the  atmosphere  and  on  the  earth's 
surface,  the  same  conditions  which  exist  in  the  Ley  den 
jar — tvvo  conducting  surfaces  insulated  by  a  dielectric ; 
the  stratum  of  least  resistance  forming  the  upper  con- 


POTENTIAL  AND  EARTH  CURRENTS.  181 

ducting  surface;  the  earth's  surface,  the  lower  one;  and 
the  dense  lower  stratum,  the  dielectric.  And,  as  in 
the  Leydeii  jar,  any  change  of  potential  in  either  sur- 
face produces  the  opposite  electric  condition  in  the 
other  surface. 

The  upper  surface,  being  insulated,  corresponds  to 
the  inner  coating;  and  the  lower  uninsulated  surface, 
to  the  outer  coating.  But  since  those  surfaces  are  of 
vast  extent,  any  limited  area  of  upper  surface  would  be 
connected  with  a  conducting  surface  at  its  outer  edges ; 
through  which  connection  electricity  would  be  repelled 
from  this  area,  or  attracted  to  it,  as  the  potential  of  the 
surface  below  it  had  a  greater  or  less  intensity.  But 
the  earth  connection,  of  the  lower  surface,  would  be 
exactly  the  same  as  that  of  the  outer  coating  of  the 
Leyden  jar. 

We  live  and  move  on  the  outer  coating  of  this  Ley- 
den  jar;  on  a  surface  practically  equipotential  within 
limited  areas ;  and  hence  do  not  perceive  electric  action 
taking  place,  no  matter  how  highly  charged  the  jar 
may  be,  except  when  the  tension  becomes  strong 
enough  to  overcome  the  resistance  of  the  dielectric,  or 
to  render  prominent  or  visible  the  action  on  either 
side  of  it: 

This  surface  then,  which  we  call  neutral,  is  really  a 
charged  surface ;  but,  like  the  outer  coating  of  a 
charged  Leyden  jar,  quiescent,  till  brought  into  action 
by  connection  with  the  inner  coating,  or  by  induction 
between  the  two. 

ASCENDING  AND  DESCENDING  CURRENTS. — We 
have  seen  how  air  currents  are  produced  by  the  action 
of  an  electric  machine,  and  how  light  bodies  vibrate 
between  electrodes  connected  with  opposite  surfaces  of 


182  ELEMENTS  OF  STATIC  ELECTRICITY. 

a  charged  Leyden  jar.  Now  since  a  constant  difference 
of  potential  is  proved  to  exist  between  the  earth's 
surface  and  the  atmosphere,  and  between  upper  and 
lower  atmospheric  strata,  we  must  conclude  that 
ascending  and  descending  currents  result  from  this 
difference :  and  that  the  clouds,  and  the  invisible  vapor 
diffused  through  the  air,  are,  like  the  air,  subject  to  this 
constant  electric  movement.  But,  there  being  also  a 
horizontal  movement,  due  to  the  winds,  the  resultant 
of  the  two  movements  is  a  series  of  curves,  ascending 
and  descending,  as  the  body  of  air  and  vapor  moves 
over  areas  of  high  or  low  potential. 

The  air  and  vapor  in  contact  with  the  earth,  becom- 
ing electrified  to  the  same  potential  as  the  earth's  sur- 
face, are  repelled,  and  attracted  upward  by  the  force 
resulting  from  difference  of  potential  in  the  stratum  of 
least  resistance  above.  Similarly  the  air  and  vapor 
above  are  repelled,  and  attracted  downward  in  conse- 
quence of  the  difference  of  potential  below. 

The  morning  and  evening  maxima,  occurring  at 
opposite  points  in  the  rational  horizon,  show  that  two 
electric  waves  traverse  the  surface  daily  from  east  to 
west,  as  the  earth  revolves  from  west  to  east.  And,  at 
points  about  equally  distant  from  these  waves,  follow 
the  two  daily  minima.  During  the  maxima  the  ascend- 
ing and  descending  currents  must  acquire  a  great 
increase,  both  in  volume  and  in  acceleration  of  move- 
ment: while  the  minima,  preceding  and  following, 
create  horizontal  movements  between  the  areas  of  high 
and  low  potential ;  producing  resultant  curves,  similar 
to  those  due  to  the  winds,  but  recurring  in  regular 
succession.  In  fact  these  currents  are  themselves 
electric  winds. 


POTENTIAL  AND  EARTH  CURRENTS.  183 

The  rarefying  of  the  air  from  heat,  at  the  time  of  the 
morning  maximum,  must  increase  and  accelerate  the  as- 
cending current,  while  its  condensation  from  cold,  at  the 
evening  maximum,  similarly  affects  the  descending  cur- 
rent ;  gravity  in  each  case  supplementing  electric  force. 

COSMEC  ELECTRIC  INFLUENCE.  —  Assuming  that 
electricity  is  a  universal  force,  acting  through  matter 
in  different  forms,  as  a  universal  medium,  it  follows 
that  electric  induction  is  universal.  Hence  induction 
between  our  planet  and  the  other  members  of  the  solar 
system,  especially  the  sun  and  moon,  must  affect  the 
electric  condition  of  the  earth  and  atmosphere. 

It  is  considered  a  well  established  principle,  that  the 
tides  are  due  to  the  attraction  of  the  sun  and  moon, 
attributed  to  gravity.  But  the  daily  electric  maxima 
and  minima  indicate  that  there  are  electric  tides,  coin- 
cident with  the  ocean  tides,  due  to  the  electric  induc- 
tion of  the  sun  and  moon :  that  an  electric  impulse 
follows  the  earth's  movement,  as  different  portions  of 
its  surface  are  successively  exposed  to  this  influence 
during  its  daily  rotation,  producing  electric  currents 
in  both  the  land  and  water  surface ;  and  perhaps  also 
tidal  waves  in  the  ocean  and  atmosphere. 

We  have  seen  that  when  a  charged  sphere  is  placed 
near  the  end  of  a  cylinder,  or  of  the  longer  axis  of  a 
spheroid,  the  electricity  of  the  cylinder  or  spheroid  is 
either  repelled  or  attracted  by  induction,  according  as 
the  potential  of  the  sphere  is  positive  or  negative,  \vith 
reference  to  that  of  the  other  body ;  and  that  this  effect 
is  intensified  when  two  charged  spheres,  at  different 
potentials,  are  placed  at  opposite  ends  of  the  cylinder, 
or  longer  axis  of  the  spheroid.  If  both  are  placed  at 
the  same  end,  the  inductive  effect  is  a  mean  between 


184  ELEMENTS  OF  STATIC  ELECTRICITY. 

the  two  effects.  But  if  one  be  placed  opposite  the 
center  of  the  cylinder  or  spheroid,  so  that  its  action  is 
at  right  angles  to  that  of  the  other,  the  intensity  of 
action  at  the  ends  is  diminished. 

In  the  sun,  moon,  and  earth,  these  conditions  are 
exactly  fulfilled  as  to  shape  and  position ;  and,  prob- 
ably also,  as  to  difference  of  potential.  The  earth  is 
an  oblate  spheroid,  whose  longer  axis  lies  east  and  west ; 
pointing  nearly  to  the  apparent  path  of  the  sun  and 
moon.  Hence  at  the  full  moon,  the  new  moon,  and 
the  quarters,  we  must  have  the  same  inductive  effects 
as  in  the  experiment  with  the  spheroid  and  the  two 
spheres.  The  earth,  at  full  moon,  is  between  the  sun 
and  moon,  and  receives  the  highest  inductive  effect. 
At  new  moon  they  are  on  the  same  side  of  it,  and 
nearly  in  line,  and  their  effect,  if  at  different  potentials, 
is  lessened :  while,  at  the  quarters,  when  the  induction 
from  each  is  at  right  angles  to  that  of  the  other,  it  is  at 
its  minimum.  Hence  we  should  expect  to  find,  as  in  the 
ocean  tides,  electric  neap  and  spring,  ebb  and  flood  tides. 

Very  little  is  known  of  the  relative  inductive  influ- 
ence of  the  sun  and  moon  on  the  earth.  Judging  from 
the  analogy  of  the  ocean  tides,  we  might  infer  that  the 
induction  of  the  moon  is  greatly  in  excess  of  that  of 
the  sun.  But  in  estimating  effects  produced  by  gravity, 
the  two  principal  factors  are  mass  and  square  of  dis- 
tance; whereas,  in  estimating  inductive  electric  effects, 
the  various  agencies  by  which  electricity  is  generated 
must  also  be  taken  into  account. 

The  nearness  of  the  moon  to  the  earth  causes  its 
effect  on  the  ocean  tides  to  be  much  greater  than  that 
of  the  sun,  though  its  mass,  as  compared  with  the  mass 
of  the  sun,  is  only  as  1  to  26,400,000.  But,  in  consid- 


POTENTIAL  AND  EARTH  CURRENTS.  185 

ering  the  electric  influence  of  the  two  bodies,  we  find 
that  the  lunar  surface  is  that  of  a  dead  world,  abso- 
lutely quiescent,  so  far  as  we  know;  while  the  solar 
surface,  to  a  great  depth,  is  in  a  state  of  the  most 
violent  agitation.  From  which  we  must  infer  a  great 
difference  of  electric  potential  in  favor  of  the  sun. 
And  observation  indicates  that  this  state  of  agitation 
affects  the  earth's  electricity ;  while  we  have  no  obser- 
vations of  electric  effects  produced  by  the  moon. 

Certain  electric  phenomena  on  the  earth  are  found 
to  coincide  with  certain  solar  phenomena.  These  con- 
sist in  violent  oscillations  of  the  magnetic  needle 
during  prominent  solar  disturbances,  indicated  by  the 
sun  spots.  And  it  is  found  that  the  periods  of  max- 
imum solar  disturbance,  which  occur  once  in  eleven 
years,  are  noted  for  corresponding  maxima  in  those 
perturbations  of 'the  magnetic  needle. 

To  make  this  clear,  it  should  be  stated  that  magnet- 
ism is  produced,  artificially,  by  the  circulation  of  an 
electric  current  around  a  conductor  capable  of  being 
magnetized,  at  right  angles  to  its  length;  as  by  a 
current  circulating  in  a  coil  of  wire,  round  a  bar  of 
iron  or  steel.  And,  conversely,  a  magnet  generates  an 
electric  current  in  such  a  coil. 

It  is  known  that  the  earth  is  a  great  natural  magnet; 
having  north  and  south  magnetic  poles,  which  exercise 
a  directive  force  on  the  magnetic  needle ;  and  it  seems 
highly  probable,  that  its  magnetism  is  the  result  of 
electric  waves,  or  impulses,  circulating  round  it  from 
east  to  west  as  has  been  shown;  giving  rise  to  electric 
currents ;  and  due  to  difference  of  temperature,  and  to 
solar  and  lunar  influences :  and  that  the  perturbations 
of  the  magnetic  needle,  coincident  with  solar  disturb- 


186  ELEMENTS  OF  STATIC  ELECTRICITY. 

ances,  are  the  result  of  corresponding  disturbances  in 
these  electric  movements. 

OBSERVATIONS  ON  TELEGRAPH  LINES. — The  tel- 
egraph affords  special  facilities  for  observing  many  of 
the  phenomena  pertaining  to  terrestrial  and  atmospheric 
electricity,  by  means  of  its  long  lines  of  nearly  uniform 
conductivity,  insulated  in  the  air,  having  earth  con- 
nections at  points  remote  from  each  other,  and  extend- 
ing, in  the  United  States,  chiefly,  either  at  right  angles 
to  the  magnetic  meridian,  or  parallel  with  it. 

These  facts  have  been  recognized;  and,  within  the 
last  five  years,  observations  have  been  made,  on  a 
limited  scale,  in  the  United  States,  and  in  Europe. 
These  observations  have  been  somewhat  desultory  and 
local ;  no  general,  extended,  well  established  system 
having  yet  been  instituted. 

During  the  fall  and  winter  of  1883-84,  a  series  of 
observations  was  made  on  a  line  belonging  to  the 
Postal  Telegraph  Co.;  extending,  at  first,  from  New 
York  City  to  Meadville,  Pa.;  509  miles  by  wire,  325 
direct;  but  subsequently  completed  to  Chicago;  1058 
miles  by  wire,  725  direct.  The  observations  from  Oct. 
18  to  Nov.  20, 1883,  were  between  New  York  and  Mead- 
ville; and  the  subsequent  observations,  which  were  con- 
tinued during  November  and  December,  1883,  and  part 
of  February,  1884,  were  between  New  York  and  Chicago. 

The  line  consisted  of  a  large  copper  wire  having  a 
steel  core ;  thus  combining  conductivity  and  strength ; 
and  the  object  of  the  observations  was  to  ascertain  the 
relations  of  the  electric  current  to  difference  of  temper- 
ature. They  were  made  daily,  at  both  ends  of  the  line, 
at  the  hours  when  it  was  least  occupied  with  other 
business,  8  to  8.30  A.M.,  5  to  5.30  and  11  to  11  30  P.M. 


POTENTIAL  AND  EARTH  CURRENTS.  187 

The  line  being  disconnected  from  the  batteries,  and 
connected  with  the  earth  at  both  ends,  the  current  was 
obtained  from  the  earth  alone,  independent  of  any  artifi- 
cial source:  and  its  strength  and  direction,  as  indicated  by 
the  galvanometer,  were  noted,  and  also  the  temperature. 

It  was  found,  that  the  general  direction  of  the  cur- 
rent was  from  a  region  of  high  to  one  of  low  temper- 
ature, though  frequent  reversals  of  current  were 
observed.  And  as  the  east,  from  longer  exposure  to 
the  sun's  heat,  would  have  a  higher  temperature  than 
$he  west,  at  the  time  of  the  morning  observation,  the 
prevailing  current,  at  this  hour,  was  found  to  be  from 
east  to  west.  As  these  conditions  of  temperature 
would  be  reversed  in  the  evening,  the  observations  at 
that  hour  showed  a  corresponding  reversal,  and  a  ' 
prevailing  west  to  east  current.  While  the  observa- 
tions near  midnight,  when  another  reversal  of  temper- 
ature is  at  hand,  showed  that  the  current  then  was 
fluctuating  and  uncertain. 

The  deflection  of  the  galvanometer  needle  varied 
from  0  to  57°;  the  morning  average  being  11.4°,  the 
evening  average  14.3°,  the  average  near  midnight  7.3°, 
and  the  general  average  11°.  The  difference  of  tem- 
perature, between  the  points  of  observation,  varied 
from  0  to  37°;  the  morning  average  being  14.5°,  the 
evening  average  9.5°,  the  average  near  midnight  10.3°, 
and  the  general  average  11.4°. 

When  the  earth  connection  was  severed,  at  either 
station,  the  current  was  reduced  to  a  minimum  ;  cor- 
responding to  the  probable  leakage  along  the  line ; 
proving  that  it  was  an  earth  current,  and  not  an 
atmospheric  current. 

If  a  similar  east  and  west  line  were  extended  rouna 


188  ELEMENTS  OF  STATIC  ELECTRICITY. 

the  globe,  we  may  reasonably  infer  that  similar  results 
would  be  observed  011  every  part  of  it;  and  hence, 
that  east  and  west  currents  are  constantly  traversing 
the  earth,  as  it  revolves  from  west  to  east. 

This  will  be  more  fully  understood,  when  we  con- 
sider, that,  during  the  diurnal  revolution  of  the  earth, 
the  sun  occupies  practically  a  fixed  position  with  ref- 
erence to  it :  so  that  from  the  earth's  heated  hemi- 
sphere, electric  currents  are  constantly  flowing,  from  a 
central  point  where  the  sun's  rays  are  vertical,  in 
opposite  directions,  towards  a  point  within  the  cooler 
hemisphere,  opposite  to  the  sun. 

But  the  diurnal  revolution  of  the  earth  brings  any 
limited  area  of  its  surface,  surrounding  an  observer, 
alternately  into  each  of  these  currents.  So  that,  while 
they  have  a  fixed  direction  with  reference  to  the  sun, 
and  to  the  earth,  as  a  whole;  they  become,  alternately, 
east  or  west  currents,  with  reference  to  such  an  area. 

From  noon  to  midnight  this  area  would  be  in  the 
west  to  east  current ;  and,  from  midnight  to  noon,  in 
the  east  to  west  current;  an  equatorial  point,  on  the 
observer's  meridian,  passing  the  point  from  which  the 
currents  diverge,  at  noon ;  and  reaching  the  point 
towards  which  they  converge,  at  midnight. 

At  both  these  hours,  the  temperature,  at  equally 
distant  points  in  the  observer's  latitude,  reaching  from 
his  position,  east  and  west  to  the  sensible  horizon,  is 
nearly  the  same :  and  the  noon  and  midnight  minima 
of  electric  potential  are  the  result. 

At  sunset  and  sunrise  the  temperature  on  similar 
quadrants  of  the  observer's  latitude,  east  and  west  of  his 
position,  attains  its  maximum  difference ;  and  the  even- 
ing and  morning  maxima  of  electric  potential  occur. 


POTENTIAL  AND  EARTH  CURRENTS.  189 

It  will  be  observed  that  while  the  observer's  position 
reaches  the  point  of  highest  temperature  at  noon,  the 
point  of  lowest  temperature  is  reached  at  sunrise.  For 
the  heating  of  any  given  area  begins  at  sunrise, 
increases  till  noon,  as  the  sun's  rays  become  more 
vertical;  and  declines  from  that  hour  till  sunset,  as 
the  rays  become  less  vertical ;  while  the  cooling  is 
constant  from  sunset  to  sunrise.  So  that  the  morning 
difference  of  temperature,  between  east  and  west 
regions,  is  greater  than  the  evening  difference ;  and  we 
should  expect  to  find  a  corresponding  increase  of  elec- 
tric potential,  at  the  morning  maximum. 

But  the  series  of  telegraphic  observations  given 
shows  the  reverse ;  which  may  result  from  the  fact  that 
the  line  on  which  the  observations  were  made,  has  the 
Atlantic  ocean  at  its  eastern  terminus,  and  the  interior 
of  the  continent  at  its  western.  And,  as  change  of 
temperature  is  much  slower  on  a  water  surface  than  on 
a  land  surface,  the  difference  of  temperature  between  the 
Atlantic  on-the  east,  receiving  the  sun's  rays  first,  and  the 
interior  on  the  west,  would  be  less  in  the  morning  than  in 
the  evening,  when  these  relative  positions  are  reversed. 

As  the  distance  between  heated  and  cooled  regions 
alternately  increases  or  diminishes  during  the  earth's 
diurnal  revolution,  electric  resistance  increases  or  di- 
minishes in  the  same  ratio,  and  increase  or  decrease  of 
current  intensity  is  a  corresponding  result :  and  electric 
maxima  and  minima,  and  also  reversal  of  current,  must 
follow  from  this  cause,  as  well  as  from  difference  or 
equality  of  temperature.  But  as  increase  or  decrease 
of  distance  is  coincident  with  increase  or  decrease  of 
difference  of  temperature,  the  two  causes  intensify  each 
other's  effects. 


CHAPTER  XIII. 
THE  ELECTRICITY  OF  THE  EARTH  AND  ATMOSPHERE. 


THE  AURORA. 

THE  relations  of  the  aurora  to  terrestrial  and  atmos- 
pheric electricity  present  a  problem  of  the  deepest  inter- 
est and  importance,  whose  satisfactory  solution  must 
render  clear  many  questions  now  involved  in  doubt  and 
obscurity.  Hence,  during  the  last  fifty  years,  it  has 
been  carefully  observed,  and  a  number  of  important  facts 
in  regard  to  it  ascertained.  The  laws  which  govern  it 
are  still  far  from  being  understood,  and  much  con- 
flict of  opinion  exists  in  regard  to  many  points;  but  its 
electric  origin  may  be  regarded  as  fully  established. 

This  phenomenon  occurs  in  zones  surrounding  the 
northern  and  southern  magnetic  poles.  And  obser- 
vations have  been  chiefly  confined  to  its  occurrence  in 
the  north.  The  northern  aurora  is  known  as  the 
aurora  borealis,  the  southern  as  the  aurora  australis, 
while  the  term  aurora  polaris,  or  simply  the  aurora,  is 
applied  to  either. 

In  the  United  States  it  is  usually  first  seen 
at  from  8  to  10  P.  M.,  though  often  beginning  much 
later :  and  it  continues  from  three  to  four  hours.  Its 
occurrence  during  the  day,  also,  is  probable ;  though 
it  can  only  be  inferred  from  coincident  effects;  the 
brilliancy  of  the  daylight  rendering  it  invisible. 


THE  AURORA.  191 

Some  of  the  great  auroras  have  been  seen  ft^r  several 
nights  in  succession  ;  their  occurrence  during  the  inter- 
vening days  also  being  highly  probable. 

AURORAL  ARCHES,  CORONA,  AND  STREAMERS. — 
It  first  appears,  usually,  as  a  low  arch  of  light,  in  the 
direction  of  the  pole,  resembling  the  dawn  of  day; 
whence  its  name,  aurora,  the  morning.  This  arch  is 
often  accompanied  by  a  low  bank  of  clouds,  lying 
under  it,  next  the  horizon.  As  the  arch  slowly  rises 
streamers  of  light,  differing  in  color,  size,  and  brilliancy, 
dart  up  through  it ;  extending  from  the  horizon  to 
a  considerable  height  above  the  arch;  their  color 
varying  from  a  pale  white  to  a  light  red;  though  yel- 
low, green,  and  blue  tints  have  also  been  observed; 
the  prevailing  tints  differing  more  or  less  in  different 
localities. 

These  streamers  appear  to  radiate  from  a  central 
region  below  the  horizon,  cutting  the  arch  vertically, 
at  right  angles,  as  shown  in  Fig.  58.  The  streamers 
sometimes  appear  to  rise  from  widely  separated  points 
in  the  horizon;  and,  as  the  aurora  increases  in  size  and 
brilliancy,  they  culminate  at  the  zenith,  as  shown  in 
Fig.  59,  forming  a  corona  of  more  or  less  prominence ; 
one  of  the  most  prominent  being  shown  in  Fig.  60. 

By  comparing  the  three  cuts,  it  will  be  seen,  that  if 
the  center  of  the  corona  shown  in  Fig.  59,  or  Fig.  60, 
were  below  the  horizon,  the  appearance  would  be  the 
same  as  in  Fig.  58.  So  that,  supposing  the  observer 
placed  below  the  horizon,  under  the  center  from  which 
the  streamers  seem  to  emanate,  he  would  see  the 
corona  above  him,  as  in  Figs.  59  and  60.  And,  con- 
versely, an  observer  in  the  latitude  of  Paris,  looking  at 
the  corona,  observed  in  latitude  70°  N.,  Fig.  60,  would 


THE  AURORA.  193 

see  only  the  upper  part  of  its  southern  half,  corre- 
sponding to  the  aurora  shown  in  Fig.  58. 

In  the  aurora  shown  in  Fig.  61,  seen  from  the  Vega, 
in  latitude  65°  N.,  we  have  an  arch  formation  without 
streamers.  A  series  of  concentric  arch  segments,  more 
or  less  perfect,  is  seen ;  the  outer  one  less  than  a  semi- 
circle, and  the  most  perfect  of  the  inner  ones  greater 
than  a  semicircle ;  the  central  one,  a  double  arch,  with 
the  nucleus  of  a  second  double  arch  above  the  junc- 
tion. From  an  inspection  of  the  figure,  it  is  evident, 
that  the  perfect  arches  would  appear  as  complete  circu- 
lar belts  to  an  observer  under  the  central  point  near 
the  horizon. 

Difference  of  longitude,  as  well  as  latitude,  must  also 
modify  the  appearance :  as  that  portion  of  the  arch 
which  appears  to  one  observer  as  its  summit,  appears 
to  another,  at  a  distant  east  or  west  point  in  the  same 
latitude,  as  its  east  or  west  base.  And,  supposing  the 
first  observer  placed  in  the  magnetic  meridian  which 
coincides  with  the  center  of  the  aurora,  the  effect  of 
perspective  would  cause  it  to  assume  a  different  appear- 
ance to  him,  from  that  seen  by  the  other  observer, 
viewing  it  from  a  different  angle.  A  streamer,  seen 
from  one  position,  would  appear  foreshortened;  while 
at  a  different  angle  it  would  appear  elongated:  to  one 
observer  it  might  appear  as  a  narrow  ray,  to  another  as 
a  broad  band. 

Hence,  we  may  infer,  that  we  see  in  the  arch,  rising 
from  the  horizon,  the  outer  edge  of  a  circular  belt 
of  electric  light,  with  its  varied  phenomena  of  arches, 
streamers,  rays,  and  coronae,  covering  a  large  area, 
parallel  to  the  earth's  surface,  and  extending,  as  it 
increases  in  size,  from  a  region  surrounding  the  pole, 


THE  AURORA.  195 

towards  the  equator :  and  that  its  different  aspects,  at 
different  times  and  places,  and  its  different  phases,  as 
seen  at  the  same  time  by  observers  at  different  points, 
are  greatly  modified  by  its  position  with  reference  to 
the  position  of  the  observer. 


Fig.  GO— Auroral  Corona  Observed  at  Bossekop,  Lat.  70°  N. 

AURORAL  MOVEMENT,  CURTAIN  FORMATION. — A 
peculiar  feature  of  the  aurora  is  the  continual  move- 
ment visible  in  every  part.  A  streamer  darts  up 
rapidly  from  the  horizon,  increasing  in  size  and  brill- 
iancy; and  as  rapidly  fades  away.  Along  one  part  of 
the  arch  a  series  of  streamers  form  in  rapid  succession, 
giving  the  impression  of  an  undulatory,  horizontal 
movement,  at  right  angles  to  the  vertical  movement  of 
the  rising  streamers :  and,  as  the  intensity  of  this  phase 
decreases,  a  similar  movement,  at  some  distant  point, 
rises  and  declines  in  a  similar  manner.  At  times  there 
occurs  a  curtain  formation,  composed  of  parallel  rays ; 
appearing  either  as  a  single  curtain,  as  shown  in  Fig. 
62,  or  as  a  series  of  curtains,  hung  one  behind  the 
other,  showing  only  their  lower  margins,  as  in  Fig.  63; 


THE  AURORA.  197 

undulatory  movements  occurring,  transverse  to  the 
apparent  vertical  position  of  the  rays,  like  the  move- 
ments of  a  banner  floating  in  the  breeze. 

This   appearance   is  doubtless   greatly  modified   by 
perspective:  the   rays    which   are    apparently  vertical, 


Fig.  02— Auroral  Curtain  Formation,  Observed  at  Bossekop,  Lat.  70°  N. 

being-  horizontal;  and  probably  emanating  from  the 
edge  of  an  arch;  producing  the  single  curtain  shown  in 
Fig.  62  ;  or  from  the  edges  of  several  concentric  arches, 
like  those  shown  in  Fig.  61 ;  producing  the  series  of 


Fig.  63— Auroral  Curtain  Formation,  Observed  at  Bossekop,  Lat.  70°  N. 

curtains  shown  in  Fig.  63.  It  is  also  evident  from  this, 
that  in  the  formation  of  coronse,  the  appearance  is 
probably  often  due  to  the  edge  of  the  arch,  with  the 
streamers  emanating  from  it,  reaching  the  zenith  of 
the  observer. 


198  ELEMENTS  OF  STATIC  ELECTRICITY. 

AURORAL  BANDS. — Sometimes  a  single  streamer 
spans  the  heavens  from  west  to  east  like  a  band.  The 
author  saw  such  a  one  at  Chicago,  Oct.  5,  1882. 
Appearing  at  about  10.30  P.  M.,  near  the  horizon,  a 
little  north  of  west,  it  extended,  within  ten  minutes, 
to  the  eastern  horizon,  passing  near  the  zenith :  and 
remained  visible  for  more  than  half  an  hour.  Its 
apparent  width  was  about  four  degrees,  and  its  color  a 
light  red. 

The  signal  service  record,  for  the  same  date,  de- 
scribes an  aurora,  "seen  generally  throughout  New 
England,  as  far  south  as  Washington,  and,  in  the 
northwest,  from  10  30  p.  M.  till  after  midnight ;  reach- 
ing an  altitude  of  90°,  and  covering  90°  of  the  horizon." 
Its  different  colors,  in  different  localities,  were  "white, 
blue,  yellow,  and  crimson.  Beams,  arches,  waves,  stream- 
ers, and  patches  of  light  were  visible ;  and,  at  Wash- 
ington, frequent  flashes  of  lightning,  at  the  edge  of  the 
dark  segment." 

HEIGHT  OF  THE  AURORA. — Great  diversity  of  opin- 
ion has  existed  in  regard  to  the  height  of  the  aurora 
above  the  earth.  A  great  altitude  has  been  assigned 
to  it  by  some,  who  argue  that  the  same  aurora  could 
not  otherwise  be  visible  to  observers  thousands  of  miles 
apart :  while  others  assign  to  it  a  low  altitude ;  main- 
taining that  these  different  observers  do  not  see  the  same 
aurora,  but  different  ones,  occurring  at  the  same  time: 
since  the  appearance  seen  by  one,  often  differs  greatly 
from  that  seen  by  another.  But,  since  differentials 
of  the  same  aurora  may  be  visible  to  different  observ- 
ers, it  is  evident,  that  one  of  low  altitude,  and  great 
extent,  might  be  seen  at  points  as  widely  remote  from 
each  other  as  the  eastern  and  western  continents ;  the 


THE  AURORA.  199 

electrified  stratum  of  the  atmosphere  surrounding  the 
polar  area,  like  a  circular  belt. 

The  weight  of  evidence  is  now  in  favor  of  the  low 
altitude ;  sixty-nine  miles  above  the  surface  being  con- 
sidered a  fair  estimate.  But  strict  accuracy  is  not 
attainable ;  since  it  is  impossible  for  any  two  observers, 
at  opposite  ends  of  a  base  line  of  sufficient  length,  to 
fix  with  certainty  on  the  same  point,  so  as  to  make  an 
angular  measurement.  But  we  can  estimate  the  prob- 
able height  at  which  atmospheric  resistance  would  be 
sufficiently  reduced  to  produce  the  auroral  phenomena  ; 
and  we  have  already  seen  that  this  plane  of  least 
resistance  must  lie  between  the  dense  strata  below 
and  the  region  of  high  vacuum  above;  both  of  which 
oppose  electric  movement.  Hence  the  height,  given 
above,  may  be  approximately  correct ;  and  yet  subject, 
doubtless,  to  variation,  resulting  from  difference  of 
atmospheric  pressure ;  low  pressure  diminishing  resist- 
ance, and  depressing  the  auroral  plane,  and  high  pres- 
sure producing  the  opposite  effect. 

GEOGRAPHICAL  POSITION  OF  THE  AURORA. — Ob- 
servation shows  that  the  aurora  is  confined  to  com- 
paratively narrow  belts.  It  is  never  seen  at  the 
equator,  and  is  rarely  visible  in  the  northern  hemi- 
sphere south  of  latitude  40°:  while  in  higher  northern 
latitudes,  it  is  seen  to  the  south  of  the  observer;  and 
decreases  in  frequency  and  brilliancy,  assuming  appar- 
ently a  more  southerly  position,  as  the  observer  moves 
farther  north. 

In  Fig.  64,  we  have  a  chart,  giving  the  results  of 
observations  made  in  the  northern  hemisphere,  by  dif- 
ferent European  observers ;  which  shows  that  this 
auroral  belt  is  about  30°  in  width.  Its  southern  limit, 


200 


ELEMENTS  OF  STATIC  ELECTRICITY. 


.  Isothasmen. 
.Magnetic  Meridians. 


Fig.  64 — Chart  showing  Isochasmen  or  Lines  of  Equal  Auroral  Frequency. 
(From  Petermann's  Mittheilungen,  20  Band,  1874— IX.) 


THE  AURORA.  201 

in  the  western  hemisphere,  is  shown  at  Lat.  22°  N.,  Long. 
75°  W.  from  Greenwich ;  and  its  northern  limit,  on 
the  same  meridian,  at  Lat.  58°  N.  In  the  eastern 
hemisphere,  its  southern  and  northern  limits,  on  the 
same  meridian,  are  between  47°  N.  and  77°  N. 

The  increased  width  and  number  of  the  lines,  towards 
the  northern  limit,  show  a  great  increase  in  the  frei 
qnency,  brilliancy,  and  duration  of  the  auroras  in  that 
region. 

It  is  also  found,  that  the  position  of  this  auroral  belt 
varies  at  different  seasons  of  the  year;  reaching  its 
southern  limit  near  the  equinoxes,  and  its  northern 
limit  near  the  solstices. 

The  results  given  in  the  above  chart  must  be  regarded 
as  approximate,  rather  than  strictly  accurate ;  as  the  data 
on  which  they  are  based  were  more  or  less  imperfect. 

CAUSES  OF  THE  AURORA. — Having  now  examined 
the  various  phases  of  auroral  phenomena,  and  their 
location,  we  are  prepared  to  investigate  more  fully  the 
causes  by  which  they  are  produced. 

The  earth  has  already  been  described  as  a  thermo- 
electric battery,  and  the  atmosphere  as  a  Ley  den  jar; 
the  one  a  generator  and  the  other  an  accumulator;  and, 
in  the  combination  of  the  two,  we  may  look  for  the 
principal  cause  of  the  aurora. 

We  have  seen  that  electric  movement  is  from  higher 
to  lower  temperature,  producing  earth  currents  on  east 
and  west  lines,  governed  by  the  earth's  rotation,  and 
by  solar  and  lunar  influence.  But  the  greater  differ- 
ence of  temperature  between  the  equatorial  and  polar 
regions  must  produce  north  and  south  currents  of  far 
greater  energy  than  these  east  and  west  currents. 

It  has  also  been  shown,  that  a  change  of  potential,  in 


202  ELEMENTS  OF  STATIC  ELECTRICITY. 

any  portion  of  the  earth's  surface,  must  produce  a 
corresponding  change  in  the  stratum  of  least  resistance, 
in  the  atmosphere  above  it;  and  that  a  transfer  of 
electricity  must  occur  between  this  electrified  atmos- 
pheric area,  and  the  surrounding  atmosphere,  lying  in 
the  same  horizontal  plane,  either  from  it  or  to  it,  as  the 
earth's  surface  below  is  positive  or  negative. 

We  have,  in  the  aurora,  the  exact  fulfillment  of  all 
these  conditions.  A  high  earth  potential,  in  the  polar 
regions,  must  result  from  the  currents  flowing  in  from 
the  warm  region ;  and  produce,  by  induction,  a  corre- 
sponding negative  potential  in  the  atmosphere.  And, 
in  the  belts  where  these  ice-bound  polar  regions  join 
the  warmer  region,  the  principal  electric  action  must 
take  place;  producing  the  auroral  arches  of  white  light : 
while  the  electricity  radiating  in  opposite  directions, 
north  and  south  from  the  arch,  produces  the  streamers, 
beams,  rays,  bands,  and  coronas ;  as  the  electric  action 
at  different  points  has  greater  or  less  intensity,  or  meets 
with  varying  resistance. 

Confirmatory  evidence  of  this  view  is  found  in  the 
fact,  shown  by  the  chart  on  page  200,  that  within  the 
torrid  and  north  frigid  zones,  where  a  comparatively 
even  temperature  exists,  the  aurora  is  not  seen ;  and 
also  in  the  shifting  position  of  the  auroral  belt  with 
change  of  temperature,  as  already  mentioned. 

The  east  and  west  earth  currents  must  also  exercise 
their  inductive  influence,  giving  rise,  probably,  to  the 
transverse  undulations  observed  in  the  streamers  and 
curtain  formations.  And  the  resultants  of  these  cur- 
rents, and  the  north  and  south  currents,  are  seen  in  the 
bands  and  streamers  which  often  assume  a  diagonal 
direction,  northwest  and  southeast,  or  otherwise. 


THE  AURORA.  203 

The  stratum,  in  which  these  phenomena  occur,  must 
have  a  certain  degree  of  thickness;  its  upper  surface 
merging  into  the  region  of  high  vacuum,  and  its  lower 
surface  into  that  of  greater  density ;  resistance  increas- 
ing upwards  and  downwards  from  a  central  plane. 
Hence,  different  phases  of  electric  action  must  occur  at 
different  altitudes;  corresponding  to  the  different  aspects 
of  electric  transmission  in  high  and  low  vacua,  seen  in 
laboratory  experiments,  as  described  in  Chapter  X : 
which  may  account  for  the  common  auroral  appearance, 
shown  in  Fig.  58,  where  the  arch  seems  to  form  a  back- 
ground for  the  streamers.  And,  as  there  is  often  a 
series  of  concentric  arches,  as  shown  in  Fig.  61,  it  is 
easy  to  see  how  streamers  might  radiate  from  one  arch, 
across  the  plane  of  another  arch,  at  a  different  altitude. 
And,  if  one  was  below,  and  the  other  above  the  horizon, 
the  appearance  would  be  the  same  as  in  Fig.  58. 

Now,  since  the  causes  here  assigned  are  in  constant 
operation,  we  may  infer  that  there  should  be  a  constant 
aurora ;  though  it  does  not  follow,  that  it  should  be 
everywhere  constantly  visible.  And  from  the  great 
number  of  auroras  observed  in  the  course  of  the  year, 
in  different  parts  of  the  auroral  belts,  especially  in  the 
northern  part  of  the  northern  belt,  it  is  reasonable  to 
infer,  that,  with  a  more  perfect  system  of  observation, 
auroras,  of  greater  or  less  magnitude,  would  be  seen,  at 
one  or  more  points,  every  night  in  the  year. 

It  is  also  probable  that  this  electric  action  may  be 
constant,  without  being  always  sufficiently  intense  to 
attract  attention :  and  that  the  aurora  is  the  result  of 
its  increased  intensity. 

Other  atmospheric  phenomena,  not  usually  recognized 
as  belonging  to  the  aurora,  may  also  be  due  to  this 


204  ELEMENTS  OF  STATIC  ELECTRICITY. 

electric  action.  The  peculiar  band  and  arch  formation 
of  cirro-stratus  clouds  often  strongly  resembling  auroral 
bands  and  arches,  has,  by  many  observers,  been  attrib- 
*ited  to  similar  electric  action ;  though  doubtless  occur- 
ring at  a  much  lower  altitude  than  that  of  the  aurora. 

The  existence  of  strong  earth  currents  during  the 
prevalence  of  auroras,  and  of  those  violent  perturba- 
tions, known  as  ''electric  storms,"  are  well  established 
facts,  proved  by  observations  on  telegraph  lines.  Dur- 
ing the  aurora  of  Feb.  4,  1872,  visible  over  an  area 
embracing  30°  of  latitude,  and  150°  of  longitude,  these 
currents  and  perturbations  were  observed  on  all  the 
lines  within  this  area,  both  land  and  submarine ;  being 
strongest  on  those  having  a  southeast  and  northwest 
direction. 

The  following  description  of  the  auroral  storm  of 
Nov.  17,  1882,  is  condensed  from  the  Signal  Service 
Reports  :  "  Beginning  a  little  before  daylight,  it  was 
known  at  first  by  its  interference  with  telegraphy. 
For  three  hours  not  a  wire  of  the  Western  Union  Tel- 
egraph Company  could  be  worked.  Late  in  the  after- 
noon, the  trouble  seemed  to  decrease ;  and,  at  night, 
there  was  a  brilliant  aurora  prevailing  over  the  eastern 
half  of  North  America,  the  Atlantic,  and  northwestern 
Europe;  and  all  telegraphic  service  was  interrupted. 
Cables  to  Europe,  and  wires  to  Chicago,  could  not  be 
worked;  annunciators  in  telephone  offices  dropped; 
the  switch-board  in  Albany,  N.  Y.,  was  ignited;  the 
switch-board  and  wires  at  Chicago  were  burned  ;  and  an 
incandescent  lamp  was  illuminated  at  St.  Paul,  Minn. 
A  message  was  sent  from  Bangor,  Me.,  to  North  Sid- 
ney, C.  B.,  710  miles,  by  the  earth  current  alone,  with- 
out the  batteries ;  the  current  being  as  strong  as  that 


AURORA.  205 

from  100  cells.  And  the  short  line  from  Boston  to 
Dedham,  ten  miles,  showed  the  disturbing  influence  as 
much  as  the  longer  lines." 

In  these  observations,  as  in  those  cited  in  Chapter  XII, 
it  has  been  found  that  whenever  the  earth  connection  is 
severed,  at  either  end  of  the  line,  the  current  immedi- 
ately ceases ;  proving  it  to  be  an  earth  current,  and 
not  a  current  in  the  atmosphere. 

The  increased  intensity  of  current,  on  lines  having  a 
southeast  and  northwest  direction,  noticed  during  the 
'aurora  of  Feb.  4,  1872,  is  confirmatory  evidence  of  the 
existence  of  resultant  currents,  as  explained  on  page 
202. 

The  hours  at  which  maximum  and  minimum  effects 
were  observed,  during  the  aurora  of  Nov.  17,  1882, 
correspond  exactly  to  the  hours  of  maxima  and  minima 
potential,  and  current  intensity,  already  cited.  A  max- 
imum having  occurred  during  the  three  morning  hours, 
beginning  just  before  daylight;  a  minimum  late  in  the 
afternoon,  and  a  maximum  again  after  sunset. 

Another  cause  of  the  aurora  is  found  in  the  move- 
ment of  warm  air  from  the  torrid  to  the  frigid  zones, 
and  of  cold  air,  at  a  lower  altitude,  from  the  frigid  zones 
to  the  tonid.  The  meeting  and  intermingling  of  these 
opposite  currents,  at  different  temperatures,  must  give 
rise  to  strong  electric  action  in  the  atmosphere,  similar 
to  that  already  described  as  taking  place  in  the  earth, 
and  coincident  with  it.  And  this  action  must  occur  in 
the  stratum  next  the  earth,  far  below  that  assigned  to 
the  aurora;  its  intensity  increasing  with  the  density  of 
the  atmosphere,  and  hence  being  greatest  at  the  earth's 
surface. 

This  becomes  evident,   when   we  consider,  that  the 


206  ELEMENTS  OF  STATIC  ELECTRICITY. 

greater  part  of  the  mass  of  the  atmosphere  lies  near  the 
earth's  surface ;  being  included,  probably,  within  the 
first  nine  miles ;  while  the  auroral  stratum  is  supposed 
to  have  an  altitude  of  sixty-nine  miles.  Hence  this 
atmospheric  electric  action  would  be  supplementary  to 
that  of  the  earth,  already  described;  and  would  have 
an  east  and  west  as  well  as  a  north  and  south  direction, 
as  described  on  page  182. 

The  influence  of  the  sun  and  moon,  already  referred 
to,  must  intensify  the  effects  produced  by  other  causes : 
so  that  we  should  expect  to  find  maximum  and  miiv 
imum  auroral  effects,  corresponding  to  an  increase  or 
decrease  of  intensity,  in  solar  or  lunar  influence.  Ob- 
servation has  shown,  that  such  an  auroral  maximum 
occurs,  during  the  recurrence,  once  in  eleven  years,  of 
the  -period  of  the  maximum  solar  disturbance;  that 
auroras  are  then  more  frequent  and  brilliant  than  at 
other  times  :  and  we  may  reasonably  infer,  that  future 
observation  will  show  the  existence  of  electric  maxima 
and  minima,  analogous  to  the  tides,  and  auroral  effects 
corresponding  to  them. 


CHAPTER  XIV. 

THE  ELECTRICITY  OF  THE  EARTH  AND  ATMOSPHERE. 


LIGHTNING  AND  THUNDER. 

FORMATION  OF  THUNDER  CLOUDS. — Our  investiga- 
tion of  this  subject  thus  far  has  been  confined  chiefly 
to  the  electricity  of  the  earth  and  its  inductive  effect 
on  the  atmosphere;  we  are- now  to  investigate  the  elec- 
tricity of  the  atmosphere  and  its  inductive  effect  on  the 
earth. 

We  have  seen,  in  the  Topler  machine,  how  electric- 
ity is  generated  by  the  mutual  friction  and  induction  of 
insulated  conductors,  put  in  motion  by  mechanical  force  ; 
and  collected  in  accumulators  which  acquire  different 
potentials,  and  between  which  a  discharge  finally  takes 
place,  attended  with  a  flash  and  report.  Something 
analogous  to  this  occurs  in  the  atmosphere.  The  clouds 
are  large  conductors,  insulated  in  the  air,  moved  by  the 
winds,  acting  inductively  on  each  other  and  on  the 
earth,  and,  in  other  respects,  fulfilling  the  same  condi- 
tions found  in  the  machine. 

As  the  vapor  forming  these  clouds  rises  from  the  earth, 
it  must  have,  when  generated,  the  same  electric  potential 
as  that  part  of  the  earth  from  which  it  rises,  and  hence 
the  same  difference  of  potential  which  has  been  shown 
to  exist  in  different  parts  of  the  earth's  surface. 

The  air  laden  with  this  rising  vapor,  moving  along  in 


208  ELEMENTS  OF  STATIC  ELECTRICITY. 

currents,  and  brought  into  contact  with  elevated  parts  of 
the  surface,  and  with  trees,  buildings,  and  other  elevated 
objects,  must  generate  electricity  by  friction,  much  in  the 
same  way  as  the  carriers  on  the  revolving  plate  of  the 
machine.  And,  as  the  vapor  forms  into  clouds,  they  be- 
come the  accumulators  of  this  electricity,  in  the  same  way 
that  it  is  accumulated  by  the  plates  and  Leyden  jars  of 
the  machine.  And  this  concentration  of  electricity  in 
the  clouds  raises  their  electric  potential;  and  makes 
them  the  nuclei  to  which  the  rising  vapor  is  attracted  in 
consequence  of  its  lower  potential. 

Each  infinitesimal  drop  of  vapor  is  a  sphere  with  its 
electric  charge  on  the  surface ;  and  as  these  drops 
coalesce,  and  form  larger  ones  in  the  cloud,  the  charge 
on  each  new  drop  accumulates  on  the  surface ;  and  as  the 
increase  of  volume  is  greatly  in  excess  of  the  increase 
of  surface,  the  electric  surface  density  must  increase 
in  nearly  the  same  ratio ;  the  volume  representing 
electric  quantity,  which  is  thus  condensed  on  a  reduced 
surface,  producing  a  corresponding  increase  of  intensity. 

Thus  as  a  large  body  of  invisible  vapor  forms  first 
into  light  fleecy  clouds;  and  these  collect  into  denser 
masses;  there  is  a  constant  reduction  of  volume,  and 
increase  of  electric  intensity;  till  the  fully  formed 
thunder  cloud  is  the  result. 

DISCHARGE  BETWEEN  CLOUDS. — Two  or  more  such 
clouds,  formed  in  different  localities,  often  many  miles 
apart,  and  electrified  in  this  manner,  must,  almost  inevi- 
tably, be  at  different  electric  potentials.  And  when  car- 
ried to\vards  each  other  by  opposite  atmospheric  currents, 
at  different  altitudes,  and  brought  within  the  sphere  of 
mutual  electric  influence,  strong  inductive  effects  are 
produced  ;  their  approach  is  accelerated  by  attraction, 


LIGHTNING  AND  THUNDER.  209 

and,  when  brought  within  proper  distance,  a  discharge 
takes  place  from  the  cloud  of  higher  to  that  of  lower 
potential :  just  as  a  similar  discharge  takes  place  be- 
tween the  sliding  electrodes  of  the  machine :  and  the 
result  is  chain  lightning,  of  which  the  spark  of  the  ma- 
chine is  an  exact  type. 

The  distance,  through  which  this  discharge  takes 
place,  depends  on  the  quantity  and  intensity  of  the 
charge,  and  the  difference  of  potential  between  the 
clouds.  It  may  be  any  distance,  from  a  few  yards  to 
several  miles.  Observation  on  discharges  between 
clouds  overhanging  fixed  localities,  as  two  mountain 
peaks,  shows  that  they  are  sometimes  from  three  to  five 
miles  or  more  in  length. 

We  have  seen  how  sparks,  eight  to  ten  inches  in 
length,  are  produced  by  the  machine ;  and  have  tested 
their  energy.  If  we  compare  such  a  discharge  to  that 
produced  between  two  clouds,  whose  magnitude  and 
potential,  as  compared  with  those  of  the  machine,  are 
almost  infinite,  we  can  form  some  adequate  conception 
of  the  enormous  energy  of  the  lightning. 

When  the  line  of  discharge  is  concealed  by  inter- 
vening clouds,  and  we  see  only  the  illumination  result- 
ing from  it,  the  phenomenon  is  known  as  sheet  light- 
ning. We  have  the  same  result,  when  the  spark  from 
the  machine,  occurring  in  a  dark  room,  is  concealed. 
Hence,  we  may  reasonably  infer,  that  the  discharge  be- 
tween the  clouds,  like  that  between  the  electrodes  of  the 
machine,  would  always  present  the  appearance  of  chain 
lightning,  if  the  line  of  discharge  were  always  visible. 

The  contorted  and  bifurcated  discharges,  known  as 
zigzag  lightning,  and  forked  lightning,  like  similar  dis- 
charges in  the  machine,  are  doubtless  due  to  differences 


210  ELEMENTS  OF  STATIC  ELECTRICITY. 

of  resistance  in  the  air,  to  the  induction  of  surrounding 
clouds,  and  to  the  mutual  repulsion  of  the  molecules  of 
air  and  vapor  within  the  line  of  discharge  ;  which,  be- 
iug  electrified  to  the  same  potential,  tend  to  separate  and 
form  resultant  lines,  under  the  influence  of  forces  act- 
ing at  right  angles  to  each  other. 

Observation  shows,  that  there  is  usually  a  succession 
of  discharges  between  the  two  clouds,  similar  to  the 
repeated  discharges  from  a  Holtz  machine :  in  which, 
after  the  initial  charge,  electricity  is  generated  by  in- 
duction alone.  This  action  begins  when  the  edges 
of  the  two  clouds,  at  different  altitudes,  approach 
within  discharging  distance,  and  come  into  vertical 
line ;  and  the  effect  of  induction  is  to  accumulate 
the  electricity  of  the  cloud  of  higher  potential  at 
the  end  nearest  to  the  other  cloud,  while  the  elec- 
tricity of  the  latter  is  repelled  to  the  remote  end  ; 
just  as  a  similar  effect  is  produced  by  the  mutual 
approach  of  two  differently  charged  conducting  plates 
or  cylinders ;  the  difference  of  potential  between  the 
adjacent  parts  being  thus  greatly  increased. 

The  discharge  produces  a  momentary  equilibrium, 
which  is  again  disturbed  by  induction,  as  larger  areas 
of  the  two  clouds  approach  more  closely:  the  residual 
becoming  the  initial  for  a  new  charge,  further  conden- 
sation taking  place,  and  fresh  supplies  of  electricity  flow- 
ing in  from  the  surrounding  atmosphere.  In  this  way 
the  series  of  discharges  continues,  till  the  clouds  unite, 
and  complete  equilibrium  takes  place. 

When  several  such  clouds,  at  different  potentials  and 
different  altitudes,  collect  in  each  other's  vicinity ;  as 
is  usually  the  case  in  a  thunder  storm  of  much  magni- 
tude ;  the  mutual  inductive  effect  is  greatly  intensified. 


LIGHTNING  AND   THUNDER.  211 

Suppose  three  clouds,  arranged  in  a  series,  end  to 
end,  and  so  graduated  as  to  potential,  that  the  central 
cloud  is  at  a  mean  between  the  other  two.  Let  a  dis- 
charge take  place  from  the  cloud  of  highest  potential 
to  the  central  one ;  a  second  discharge  must  quickly 
follow,  from  the  central  cloud  to  the  one  of  lowest  po- 
tential :  since  the  first  discharge  has  greatly  increased 
their  difference  of  potential.  This  second  discharge 
would  renew  the  difference  of  potential  between  the 
first  and  central  clouds,  and  prepare  the  way  for  another 
series  of  similar  discharges. 

The  most  careless  observer  cannot  fail  to  have  noticed 
such  series  of  discharges,  following  each  other  in 
rapid  succession,  in  different  parts  of  the  sky,  during  a 
violent  thunder  storm. 

Observation  also  shows,  that  during  a  thunder  show- 
er, there  is  always  an  increase  of  rain-fall,  and  an  en- 
largement of  the  drops,  within  a  few  seconds  after  each 
electric  discharge ;  the  time  being  just  sufficient  for 
the  rain  to  descend,  if  it  left  the  cloud  at  the  moment 
of  the  discharge.  From  which  we  may  infer,  that  con- 
densation is  a  result  of  the  discharge  ;  that,  in  the  mo- 
mentary equilibrium  which  follows  it,  the  small  drops, 
which  were  before  kept  apart  by  mutual  repulsion, 
from  being  highly  charged  and  at  the  same  potential, 
now  coalesce,  and  form  the  large  drops  ;  which,  being 
too  heavy  to  be  sustained  in  the  atmosphere,  fall. 

THUNDER. — As  the  spark  from  the  machine  is  the 
type  of  lightning,  so  the  snap  represents  tlmnder ; 
which  is  undoubtedly  due  to  the  same  cause — the  sud- 
den and  intense  vibratory  motion  of  the  air,  in  the  line 
of  discharge,  producing  violent  undulations  in  the  sur- 
rounding air.  A  cause  which  will  appear  sufficiently 


212  ELEMENTS   OF  STATIC  ELECTRICITY. 

adequate,  when  we  consider  the  results  which  must  fol- 
low from  the  rush  of  the  enormous  energy  of  a  thunder 
cloud,  along  a  line,  perhaps  five  miles  in  length,  in  an 
infinitesimal  fraction  of  a  second. 

And  here,  as  in  the  case  of  the  spark,  it  is  quite  un- 
necessary to  suppose  the  passage  of  any  material  sub- 
stance through  the  air,  producing  partial  vacuum  and 
collapse,  or  the  occurrence  of  anything  in  the  nature  of 
an  explosion,  producing  similar  results.  It  is  more  in 
accordance  with  the  known  laws  of  electric  movement, 
to  suppose  that  the  electric  energy  has  used  the  air  as 
the  medium  in  which  to  travel;  and  thus  produced  the 
vibratory  motion. 

Common  observation  shows,  that  in  explosions  where 
the  expenditure  of  energy  must  often  be  far  less  than 
in  the  electric  discharge  between  clouds,  the  vacuum 
and  collapse  shatter  window-glass  in  the  vicinity;  while 
the  heaviest  thunder  produces  only  a  slight  tremor  in 
adjacent  buildings ;  proving  that  such  vacuum  and  col- 
lapse cannot  result  from  an  electric  discharge. 

The  succession  of  reports  accompanied  by  a  continu- 
ous rumble,  heard  so  frequently  during  a  thunder  storm, 
has  been  considered,  by  some  observers,  as  a  series  of 
echoes  from  a  single  report;  and  by  others,  as  a  num- 
ber of  separate  reports,  from  discharges  occurring  si- 
multaneous]}T,  at  different  distances  from  the  observer, 
and  heard  in  the  order  of  their  distance. 

An  echo  requires  the  intervention  of  an  extended 
surface,  as  a  wall  or  its  equivalent;  and  observation 
shows,  that  the  under  surface  of  a  dense  thunder  cloud 
is  of  this  character,  being  remarkably  uniform,  though 
its  upper  surface  may  be  quite  the  reverse  :  and  it  is  also 
evident,  that  this  under  surface,  resting  on  tho  denser 


LIGHTNING  AND   THUNDER.  \^O^    Zl3 

£L 

strata  of  air,  and  sustaining  the  weight  of  the  ~ 
air  and  vapor  above,  must  have  greater  density  than 
the  upper  surface.     Hence   we    may    reasonably   infer, 
that  this  surface,  and  that  of  the  earth  below  it,  fulfill 
the  conditions  necessary  for  a  series  of  echoes. 

The  hypothesis  of  simultaneous  discharges,  at  differ- 
ent distances,  may  also  be  true  in  certain  instances :  as 
it  is  quite  possible  that  such  simultaneous  discharges 
frequently  occur.  But  the  succession  of  reports,  often 
following  each  other  with  marked  regularity,  and  steadi- 
ly decreasing  in  volume  and  intensity,  is  not  fully  ex- 
plained by  this  hypothesis,  while  it  is  entirely  in  ac- 
cordance with  the  character  of  a  series  of  echoes. 

The  re-adjustment  of  electric  energy  between  differ- 
ent parts  of  a  large  cloud,  which  must  follow  the  pri- 
mary discharge,  gives  rise  to  numerous  minor  discharges; 
whose  sound,  mingling  with  that  from  the  larger  air 
waves,  causes  the  rumble ;  analogous  to  the  crackling 
sound  from  similar  minor  discharges  in  the  machine.  A 
premonitory  rumble,  from  a  similar  cause,  often  precedes 
the  heavier  discharge ;  just  as  the  crackling  precedes 
the  discharge  of  the  machine. 

If  the  cloud  were  a  perfectly  homogeneous  conductor, 
like  a  metal  cylinder,  this  could  not  occur.  But  as  it 
is  a  mass  of  vapor,  composed  of  drops  insulated  from 
each  other  by  air  spaces,  each  particular  drop  having  its 
own  electric  charge ;  and  different  parts  of  the  cloud 
having  different  densities,  and  hence  differing  in  con- 
ductivity and  resistance;  and  condensation,  with  increase 
of  potential,  following  the  discharge,  as  already  shown, 
such  minor  discharges,  with  the  accompanying  roar  and 
rumble,  are  inevitable.  Also  the  development  of  the 
residual,  after  the  primary  discharge,  which,  in  a  large 


214  ELEMENTS  OF  STATIC  ELECTRICITY. 

cloud,  must  iii  itself  have  great  energy,  greatly  intensi- 
fies these  effects. 

DISCHARGE  FROM  THE  CLOUDS  TO  THE  EARTH.— 
We  have  already  seen  that  the  potential  of  the  atmos- 
phere, and  hence  of  the  clouds,  is  almost  invariably 
positive  with  reference  to  that  of  the  earth.  Hence  the 
earth's  surface  under  a  thunder  cloud,  and  all  objects 
connected  with  it,  become  negatively  electrified  by  in- 
duction, to  the  same  degree  that  the  cloud  is  positive ; 
electricity,  equal  to  the  charge  of  the  cloud,  being  re- 
pelled from  the  earth's  surface  to  its  interior.  A  result  of 
this  difference  of  potential  is  a  strong  attraction  between 
the  earth  and  cloud,  by  which  the  cloud  is  drawn  towards 
the  earth;  and,  unless  its  potential  is  reduced  by  discharge 
into  another  cloud,  a  discharge  to  the  earth  is  inevitable, 
whenever,  from  reduction  of  distance,  the  resistance  of 
the  air  becomes  less  than  the  electric  tension  of  the  cloud. 

When  there  are  two  clouds  at  different  altitudes,  and 
a  discharge  takes  place  from  the  upper  to  the  lower 
cloud,  the  difference  of  potential  between  the  latter  and 
the  earth,  being  thus  increased,  the  liability  of  a  dis- 
charge from  it  to  the  earth  is  increased  in  the  same  ratio. 

If  there  are  elevated  objects,  such  as  trees  and  build- 
ings, on  the  surface  below,  the  resistance  between  them 
and  the  cloud  is  less  than  that  of  the  surrounding  flat 
surface ;  not  only  on  account  of  reduced  distance,  but 
also  on  account  of  the  points  and  angles  which  they 
present.  Hence,  we  find,  that  trees,  flag-staffs,  tele- 
graph poles,  church  spires,  chimneys,  and  projecting 
corners  of  roofs  are  much  more  frequently  struck  by 
lightning  than  flat  surfaces. 

Good  conductors,  such  as  tin  gutters,  metal  cornices, 
and  ornamental  iron  work,  also  offer  far  less  resistance 


LIGHTNING  AND   THUNDER.  215 

than  imperfect  conductors,  like  wood,  brick,  and  stone; 
both  from  their  superior  conduct!  vity,  and  their  projecting 
edges  and  points  ;  and  when  connected  with  a  building 
and  not  connected  by  a  metallic  conductor  with  the  earth, 
greatly  increase  the  liability  of  the  building,  both  to  re- 
ceive the  electric  discharge,  and  to  sustain  injury  from  it, 
by  making  the  building  its  terminus  instead  of  the  earth. 

DISCHARGE  FROM  THE  EARTH  TO  THE  CLOUDS. — 
As  already  shown,  the  electricity  of  a  large  cloud,  like 
that  of  a  cylinder,  may  be  so  distributed  by  the  prox- 
imity of  one  end  to  another  cloud,  at  a  lower  potential, 
or  to  an  elevated  portion  of  the  earth's  surface,  that  the 
potential  of  this  end  shall  be  higher  than  that  of  the 
remote  end.  The  potential  of  the  earth's  surface,  be- 
neath it,  must  also  be  similarly  affected  by  induction,  in 
reverse  order;  being  negative  where  the  cloud  is  positive, 
and  positive  where  the  cloud  is  negative.  If,  under 
these  circumstances,  the  difference  of  potential  between 
the  negative  end  of  the  cloud  and  the  earth  becomes 
greater  than  the  resistance  of  the  air,  a  discharge  from 
the  earth  to  the  cloud  must  occur;  the  discharge  in  this, 
as  in  all  other  cases,  being  from  higher  to  lower  potential. 

These  conditions  are  similar  to  those  of  the  three 
clouds  already  referred  to :  so  that  a  discharge  from 
the  positive  end  to  another  cloud,  or  to  the  earth,  may 
increase  the  difference  of  potential  between  earth  and 
cloud  at  the  negative  end. 

The  resistance  of  the  earth,  also,  over  such  an  exten- 
sive area,  retards  the  restoration  of  surface  equilibrium 
after  the  discharge  from  the  positive  end;  and  increases 
the  liability  of  the  return  discharge  from  the  earth  to 
the  cloud,  in  the  ratio  of  this  resistance  to  that  of  the 
vapor  of  the  cloud. 


216  ELEMENTS  OF  STATIC  ELECTRICITY. 

In  this  case,  as  in  that  of  a  discharge  from  the  clouds 
to  the  earth,  elevated  objects  reduce  the  resistance,  es- 
pecially if  they  are  good  conductors,  or  furnished  with 
sharp  angles  or  points;  and  become  the  electrodes 
through  which  the  discharge  takes  place. 

LIGHTNING  RODS. — Franklin  first  proposed  the 
lightning  rod.  The  identity  of  lightning  and  elec- 
tricity, strange  to  say,  was  unknown,  till,  by  the  erec- 
tion of  a  metal  rod  at  his  suggestion,  and  subsequently 
by  his  well  known  kite  experiment,  sparks  were  drawn 
from  the  cloud,  Leyden  jars  charged,  and  various  similar 
laboratory  experiments,  previously  known  to  electric 
science,  performed  by  means  of  atmospheric  electricity. 

The  first  lightning  rod  was  erected,  May  10,  1752,  a 
month  previous  to  the  kite  experiment,  by  M.  Dalibard, 
in  France,  according  to  the  plan  proposed  by  Franklin 
for  testing  the  identity  of  lightning  and  electricity: 
and  sparks  similar  to  those  from  the  electric  machine 
were  drawn  from  it. 

The  identity  of  lightning  and  electricity  having  been 
established,  Franklin  showed  how  the  rod  could  be  used 
as  a  means  of  protecting  buildings.  The  result  is  the 
lightning  rod,  as  we  now  have  it,  in  its  numerous  forms. 
And  though  ignorance,  greed,  and  dishonesty  have  cast 
their  shadow  upon  it,  yet  thousands  of  well  con- 
structed rods,  standing  as  the  silent  guardians  of  life 
and  property,  sufficiently  attest  its  value. 

The  proper  construction  of  lightning  rods  was  re- 
cently investigated  by  a  conference  of  leading  English 
scientists,  specially  appointed  for  that  purpose  :  among 
whom  were  several  eminent  electricians.  And,  after 
three  years  of  thorough  investigation,  during  which 
practical  information  was  collected  from  all  parts  of 


LIGHTNING  AND   THUNDER.  217 

the  world,  a  code  of  rules  for  the  construction  and 
erection  of  lightning  rods,  or  conductors,  was  adopted 
December  14, 1881;  which  is  substantially  as  follows: — 

RULES  FOR  THE  CONSTRUCTION^  AND  ERECTION  OF 
LIGHTNING  CONDUCTORS. 

POINTS  AND  UPPER  TERMINALS. — As  the  point  of 
the  upper  terminal,  from  its  peculiarly  exposed  position, 
is  liable  to  be  fused  by  a  heavy  charge,  it  should  not 
be  sharper  than  a  cone  whose  height  is  equal  to  the 
radius  of  its  base.  But,  to  secure  the  peculiar  advan- 
tages derived  from  sharp  points,  three  or  four  such 
points  made  of  copper,  each  about  six  inches  long, 
should  be  attached  to  a  copper  ring ;  which  should  be 
screwed  or  soldered  to  the  terminal,  about  twelve  inches 
below  its  highest  point.  And  all  points  should  be  so 
platinized,  gilded,  or  nickel-plated,  as  to  resist  oxidation. 

The  number  of  terminals  required,  their  height  above 
the  building,  and  the  number  of  conductors  connected 
with  them,  depends  on  the  size  and  style  of  the  build- 
ing, and  the  conductivity  of  the  material  of  which  it  is 
constructed. 

All  elevated  parts,  such  as  turrets  and  spires,  should 
be  protected  by  terminals :  and  especially  chimneys, 
whose  liability  to  receive  a  discharge  is  greatly  increased 
by  the  heated  air  and  soot. 

Factory  chimneys  should  have  a  copper  band  round  the 
top;  with  stout,  sharp,  copper  points,  each  about  twelve 
inches  long,  projecting  from  it  at  intervals  of  two  or  three 
feet,  and  specially  guarded  against  oxidation.  And  the 
conductor,  attached  to  this  band,  should  be  attached  to 
all  bands  and  metallic  masses  in  or  near  the  chimney. 

SPACE  PROTECTED. — No  definite  rule  can  be  given 


218  ELEMENTS   OF  STATIC  ELECTRICITY. 

as  to  the  space  protected  by  a  conductor ;  as  opinion 
and  practice  vary  in  regard  to  it:  but  there  is  no  well 
authenticated  instance  of  a  building  furnished  with  a 
property  constructed  conductor,  having  been  injured 
by  lightning  within  a  conical  space,  having  the  point 
of  the  upper  terminal  for  its  apex,  and  the  radius  of 
whose  base  equaled  the  height  of  the  conductor. 

ATTACHMENT  TO  BUILDING. — The  evidence  against 
the  use  of  glass  or  other  material,  in  order  to  insulate 
the  conductor,  is  overwhelming ;  and  insulation  may 
be  regarded  as  unnecessary  and  mischievous.  The. 
attachment  to  the  building  should  be  made  with  metal 
fastenings ;  which  should  be  of  the  same  metal  as  the 
conductor  itself,  to  prevent  corrosion  from  galvanic 
action.  They  should  be  of  adequate  strength :  and 
each  should  support  its  proper  proportion  of  the  weight. 
They  should  not  compress  or  distort  the  conductor;  and 
should  allow  free  play  for  its  expansion  and  contraction. 

As  far  as  practicable,  it  is  desirable  that  conductors 
be  connected  with  extensive  masses  of  metal  belonging 
to  the  building,  both  internal  and  external;  except 
soft  metal  pipes,  which,  from  low  conductivity  for  heat 
and  electricity,  are  liable  to  fusion.  Gas-pipes,  es- 
pecially, should  not  be  so  connected  on  account  of 
liability  to  ignition  of  the  gas  by  an  electric  spark, 
resulting  from  fusion  of  the  pipe,  or  from  bad  joints : 
but  the  inlet  and  outlet  pipes  of  large  gas  meters  should 
always  be  electrically  connected  with  each  other,  as  a 
protection  against  such  accidents  from  the  electric 
resistance  of  joints;  which  is  sometimes  greatly  in- 
creased by  india-rubber  packing. 

Church  bells,  inside  well  protected  steeples,  need 
not  be  connected  with  the  conductor. 


LIGHTNING  AND    THUNDER.  219 

ORNAMENTAL  IRON  WORK.  —  All  vanes,  finials, 
ridge  iron  work,  and  similar  ornamental  metal  work, 
should  be  connected  with  the  conductor :  and  it  is  not 
absolutely  necessary  to  use  any  other  point  than  that 
afforded  by  such  ornamental  work ;  provided  the  con- 
nection be  perfect,  and  the  mass  of  iron  considerable. 
As,  however,  there  is  risk  of  derangement  through  re- 
pairs, it  is  safer  to  have  an  independent  upper-  terminal. 

MATERIAL  FOR  CONDUCTOR. — The  best  material  for 
a  conductor  is  copper ;  its  weight  not  less  than  six 
ounces  per  foot  run ;  and  its  conductivity  not  less  than 
ninety  per  cent,  of  that  of  pure  copper.  It  may  be 
used  either  in  the  form  of  tape,  or  of  wire  cable,  in 
which  110  wire  should  be  less  than  No.  12  B.  W.  G. 
Iron  may  be  used,  but  its  weight  should  not  be  less 
than  2i  poundsj^er  foot  run.  And  all  iron  conductors, 
whether  galvanized  or  not,  should  be  painted,  as  a 
protection  against  oxidation.  Copper  conductors  may  be 
painted  or  not  according  to  architectural  requirements. 

FORM  OF  CONDUCTOR. — The  form  of  the  conductor 
does  not  seriously  affect  its  conductivity :  and  great  ex- 
tent of  surface  in  proportion  to  mass  is  not  essential :  but 
sectional  area  of  mass  is  highly  essen tial,and  should  al  ways 
be  sufficient  to  carry  the  heaviest  charge  without  dan- 
ger of  fusion  of  the  conductor,  or  division  of  the  current. 

The  rod  is  desirable  for  long  upper  terminals,  on 
account  of  its  rigidity ;  but  the  necessity  of  frequent 
joints,  and  the  difficulty  of  avoiding  disfigurement  of 
the  building,  are  serious  objections  to  its  use  for  the 
body  of  the  conductor. 

Tubes  are  liable  to  the  same  objections ;  their  larger 
diameter,  and  the  collars  necessary  for  their  joints,  ren- 
dering them  more  conspicuous  and  undesirable. 


220  ELEMENTS  OF  STATIC  ELECTRICITY. 

Twisted  wire  cables  have  the  advantage  of  compara- 
tive freedom  from  joints  ;  but  their  interstices  afford  a 
lodgment  for  smoke,  dirt,  and  water;  especially  if 
small  wires  are  used:  which  are  also  less  capable  of 
resisting  oxidation  than  large  wires. 

Tape  has  the  special  advantages  of  requiring  but  few 
joints;  of  their  being  easily  made,  where  necessary; 
and  of  being  flat  and  flexible,  so  that  it  can  be  adapted 
to  the  outlines  of  a  building,  or  countersunk  in  it  and 
painted  over,  so  as  not  to  be  conspicuous. 

Conductors  should  not  be  bent  abruptly  round  sharp 
corners :  and  in  no  case  should  the  length  of  conductor 
between  the  two  points  of  a  bend  be  more  than  one- 
half  greater  than  the  straight  line  joining  them.  When> 
practicable,  the  conductor  may  pass  straight  through  a 
projection;  the  hole  being  made  large. enough  to  allow 
it  to  pass  freely,  without  compression. 
x  The  reasons  for  these  precautions  are  found  in  the 
liability  to  discharge  from  a  sharp  angle,  or  across  a 
short  space  in  a  bend. 

JOINTS. — The  most  fruitful  source  of  danger  in  con- 
ductors is  from  bad  joints.  Screwed,  scarfed,  or  riveted 
joints,  however  well  made,  are  certain  to  rust  and  cor- 
rode in  time  ;  introducing  nodes  of  resistance,  at  which 
the  electric  charge  is  liable  either  to  fuse  the  conductor, 
or  to  leave  it  and  enter  the  building. 

No  joint  is  electrically  perfect  that  is  not  metallically 
continuous,  and  as  absolutely  free  from  resistance  as 
any  other  part  of  the  conductor :  and  careful  soldering, 
in  addition  to  the  screwing,  scarfing,  or  riveting,  is  the 
only  certain  means  of  securing  this,  which  has  borne 
the  test  of  experience. 

EARTH  CONNECTION. — A  good  earth  connection,  for 


LIGHTNING  AND   THUNDER.  221 

the  lower  terminal,  is  of  the  utmost  importance ;  and 
in  a  majority  of  cases  of  injury  to  buildings  from  badly 
constructed  conductors,  such  injury  is  traceable  to 
imperfect  earth  terminals. 

The  terminal  should  connect  with  damp  earth,  at  a 
sufficient  depth  below  the  surface,  to  insure  permanent 
dampness,  and  hence  permanent  conductivity.  •  And, 
to  render  this  connection  more  complete,  it  should 
bifurcate  below  the  surface  ;  and  be  connected  by  sol- 
dering, with  a  mass  of  metal,  buried  in  the  earth.  The 
hole,  in  which  this  mass  is  buried,  should  be  filled  to  the 
surrace  with  cinders  or  coke,  to  facilitate  the  percolation 
of  water;  and  any  available  drainage  of  pure  water, 
from  rain  water  pipes  or  otherwise,  connected  with  it. 

The  metal  mass  may  be  of  copper  or  galvanized  iron, 
having  about  eighteen  square  feet  of  surface.  And 
where  permanently  damp  earth  is  not  available,  it 
should  consist  of  three  or  four  hundred  pounds  of  iron. 

Where  the  use  of  large  iron  water  or  gas  mains  is 
available,  a  connection  by  a  copper  strip,  can  be  made 
with  them ;  no  risk  being  incurred  by  such  connection, 
as  in  the  case  of  internal  supply  pipes. 

INSPECTION. — Periodical  inspection,  and  careful  elec- 
tric testing,  are  requisite  to  maintain  the  system  in 
efficient  order ;  as  points  may  corrode  or  become  fused, 
joints  become  electrically  imperfect,  connections  be- 
come severed  above  or  below  ground,  or  other  im- 
perfections occur,  from  alterations  in  the  building,  and 
the  carelessness  or  ignorance  of  occupants  or  workmen. 


The  author  has,  on  his  house,  a  copper  tape  conductor, 
constructed  in  accordance  with  these  principles,  and 
erected  twenty-three  years  ago  ;  and  neither  the  house, 


222  ELEMENTS   OF  STATIC  ELECTRICITY. 

nor  the  conductor,  has  ever  received  the  slightest  injury 
from  lightning;  while  numerous  instances  of  damage 
to  buildings  and  conductors  have  occurred  in  the  vicin- 
ity. Which,  considering  the  length  of  time,  the 
exposed  position,  and  the  repeated  thunder  storms  of 
great  severity,  which  have  occurred,  is  strong  negative 
evidence  of  the  value  of  the  conductor,  and  the  correct- 
ness of  the  rules  here  given. 

SILENT  DISCHARGE. — The  protection  afforded  by  a 
lightning  conductor  does  not  consist,  so  much,  in  its 
being  the  avenue  by  which  a  destructive  discharge  may 
pass  harmlessly  between  the  earth  and  cloud ;  as  in 
preventing  its  occurrence,  by  a  gradual,  silent  discharge 
through  the  points  of  the  conductor;  by  which  the 
accumulated  energy  is  reduced,  before  it  can  acquire 
sufficient  tension  to  overcome  the  resistance  of  the  air, 
and  produce  a  full,  sudden,  disruptive  discharge. 

This  is  strikingly  illustrated  by  the  gradual,  silent 
discharge  of  a  large,  powerfully  charged  Leyden  bat- 
tery, through  the  point  of  a  cambric  needle ;  and  is 
confirmed  by  the  brush  discharge,  often  observed, 
during  thunder  storms,  on  the  points  of  lightning 
conductors,  and  on  the  tips  of  the  masts  and  yard-arms 
of  ships. 

As  a  building  must  be  regarded,  electrically,  as  an 
elevated  part  of  the  earth's  surface,  the  importance  of 
as  perfect  an  electric  connection  between  it  and  the 
conductor,  as  practicable,  is  apparent,  in  order  to 
secure  the  full  benefit  of  protection  in  the  manner  de- 
scribed;  which  is  impaired  by  the  resistance  caused  by 
the  use  of  insulators. 

It  is  also  apparent,  that  the  conductor  affords  equal 
protection  whether  the  discharge  is  from  the  cloud  to 


LIGHTNING  AND   THUNDER.  223 

the  earth,  or  from  the  earth  to  the  cloud ;  as  in  either 
case,  the  discharge  will  follow  the  path  of  least  resist- 
ance ;  which  is  always  through  the  conductor,  when 
properly  constructed. 

HEAT  LIGHTNING. — The  phenomenon,  known  as 
heat  lightning,  is  probably  nothing  more  than  the  or- 
dinary electric  discharge  from  clouds  invisible  to  the 
observer,  and  so  distant  that  the  thunder  is  inaudible. 
Such  lightning  is  generally  observed  at  night,  near  the 
horizon ;  and  close  observation  will  show,  either  the 
existence  of  clouds,  indistinctly  visible  in  the  darkness, 
or  the  probability  of  the  discharge  occurring  from 
clouds  below  the  horizon. 

Its  existence,  independent  of  clouds,  is  claimed  from 
the  fact,  that  it  has  been  observed  when  no  thunder 
storm  had  occurred  within  a  radius  of  one  hundred 
miles.  But,  not  only  lightning,  but  clouds  are  often 
visible  at  greater  distances.  On  the  level  surface 
round  Chicago,  the  author  has  frequently  observed 
heavy  thunder  storms,  eighty  miles  distant,  as  shown 
by  subsequent  reports,  when  both  clouds  and  lightning 
were  distinctly  visible,  though  the  thunder  was  not 
audible. 

TORNADOES. — As  an  electric  origin  has  been  claimed 
for  tornadoes,  it  is  proper  to  remark,  in  conclusion, 
that  recent  investigation  has  demonstrated  that  they 
are  chiefly  due  to  currents  of  air,  generated  by  differ- 
ences of  atmospheric  temperature  and  pressure,  and 
modified  by  other  causes:  and  while  electricity  may 
intensify  their  force,  it  cannot  be  considered  as  their 
primary  cause. 


224  ELEMENTS  OF  STATIC  ELECTRICITY. 

NOTE   REFERRED    TO    ON    PAGE   118. 

The  brush  from  K  makes  its  appearance  first,  and 
increases  in  length  till  the  brush  from  V  appears;  after 
which  it  decreases  in  the  same  ratio  as  the  brush  from 
V  increases,  till  the  discharge  occurs,  when  both  dis- 
appear. This  is  sufficiently  explained  by  increase  and 
decrease  of  difference  of  potential  at  different  points. 
As  the  potential  of  the  revolving  plate  A  increases,  the 
difference  of  potential  between  the  inside  coating  of  the 
jar  (7,  and  that  part  of  A  which  receives  the  charge 
from  it  through  the  comb  K,  decreases,  as  indicated  by 
the  decrease  in  brush-length,  till  the  potential  of  both 
is  the  same,  when  the  brush  disappears. 

In  like  manner  the  potential  of  that  part  of  the  plate 
A,  passing  the  comb  L,  continues  to  increase  till  it 
equals  the  potential  of  the  inside  coating  of  the  jar  D; 
and  this  charged  surface,  passing  on  to  the  comb  H,  the 
surplus  of  charge  which  D,  from  increase  of  potential 
rejects,  escapes  through  II  to  the  comb  V,  and  from  V 
to  that  part  of  the  plate  A  between  V  and  K,  as  indi- 
cated by  the  increase  of  brush-length  from  V. 

This  process  is  greatly  intensified  by  the  inductive 
effect  of  the  high  potential  of  the  lower  part  of  inductor 
T,  and  low  potential  of  the  upper  part  of  inductor  Jf, 
by  which  electricity  is  repelled  from  the  corresponding 
lower  part  of  the  plate  A  to  its  corresponding  upper 
part. 


INDEX. 


Absolute  Electrometer,  Thomson's,  161-169. 

Accumulators,  72-91. 

Amber,  1. 

Atmosphere,  the,  as  a  Leyden  jar,  180, 181. 

Atmospheric  potential,  177-180. 

strata,  difference  of  potential  between, 
179, 180. 

currents,  181-183. 

Attraction  and  repulsion,  1-4, 15,  40-42. 
Aurora,  the,  190-206. 

,  height  of  the,  198,  199. 

,  geographical  position  of  the,  199-201. 

,  causes  of  the,  201-206. 

,  tubes,  146, 147. 

Auroral  arches,  coronas,  and  streamers,  191- 
195. 

movement,  curtain  formation,  195-197. 

bands,  198. 


Bag  experiment,  60. 
Balanced  rod,  the,  2. 
Bath,  electric,  142, 143. 
Battery,  the  Leyden.  79,  80. 
Bells,  electric,  102, 103, 125, 126. 
Brush  discharge,  117, 118,  134, 137. 


Charge  denned,  22. 

,  multiplication  of,  in  Topler  machine, 
121,  122. 

,  variation  of,  67. 

Charged  surfaces,  formulae  for,  167. 
Chime,  electric,  for  frictional  machine,  102, 
103. 

,  for  Topler  machine.  125. 126. 
Condensation,  surface,  55-58. 
Condi-users,  74. 

15 


Conductivity  for  heat  and  electricity  com- 
pared, 37,  38. 
Conductors  and  non-conductors,  4-6. 

,  hollow,  58,  59-66. 
Conservation  of  energy,  the,  23-26. 
Convection,  66,  67. 
Cosmic  electric  influence,  183-186. 
Coulomb's  torsion  balance,  156-161. 
Currents,  atmospheric,  181-183. 

,  earth,  186-189,  204,  205. 
Cylinder,  electrified,  48,  69. 

Avith  points,  70. 


Dielectric  denned,  50. 

,  required  thickness  of,  74. 
Disc,  electrified,  71. 
Discharge,  appai-ent  time  of,  126-128. 

,  brush,  117, 118, 134, 137. 

between  clouds,  208-211. 

from  the  clouds  to  the  earth,  214,  215. 

from  the  earth  to  the  clouds,  125, 126. 

,  disruptive,  88. 

,  silent,  89. 

,  spontaneous,  88. 

through  book,  81-84. 
Discharger,  76. 

,  universal,  87,  88. 
Dual  theory,  the,  40-42. 


Earth  currents,  186-189,  204,  205. 
Ebonite,  1,  6,  53,  54. 
Electricity,  the  nature  of,  23-42. 

of  the  earth  and  atmosphere,  175-223. 

generated  by  the  friction  of  metals,  132, 

133. 

Electrics,  4. 
Electric  bath,  142, 143. 


226 


INDEX. 


Electric  movement,  13-16. 

potential,  10-11. 

transmission  in  vacua,  146-154. 

wind,  104, 105,  143. 
Electrometers,  155-174. 

,  attracted  disc,  161. 
Electrometer,  Thomson's  absolute,  161-1C9. 

,  mode  of  using  the  absolute,  166-169. 

,  Thomson's  quadrant,  109-174. 

,  mode  of  using  the  quadrant,  173, 174. 
Klectrophorus,  the,  92-96. 
Electroscope,  the  gold  leaf,  16-18. 

,  the  pith  ball,  2  3. 

,  charged  by  induction,  44. 
Energy,  the  conservation  of,  23-26. 

,  radiant,  31. 
Ether,  31-33. 
Equipotential,  55. 
Experiments  with  the  Topler  machine  ,125-145. 


F 


Farad  iy's  hollow  cube,  65. 
Faradic  current,  141. 
Figures,  Lichtenberg's,  89-91. 
Force,  1. 

,  lines  of,  55. 
Form,  influence  of,  67. 
Formulae  for  charged  surfaces,  167. 

,  application  of,  to  measurement  by  elec- 
trometer, 167-169. 
Fracture  of  Leyden  jar,  88,  140. 
Friction,  mutual  effects  of,  18-21. 
Frictional  electricity,  8,  9. 

machine,  96-100. 


G 


Gauge,  idiostatic,  for  electrometer,  63. 
Gas  lighting,  143-145. 
Geissler  tubes,  147,  148. 
Generators,  electric,  92-124. 
Glass  for  Leydeu  jars,  77. 

illuminated  by  electricity,  151. 

,  required  thickness  of,  for  insulation,  74. 

,  specific  inductive  capacity  of,  53,  54. 
Gravity  and  electiicity  compared,  13, 14. 
Gunpowder,  method  of  exploding  by  elec- 
tricity, 87. 


IT 


Heat  and  electricity  compared,  13, 14,  33,  37, 


Heat,  light,  and  electricity  compared,  26-31. 
Heat  lightning,  223. 


Heating  effects  of  electricity  in  high  vacua, 

153,  154. 

Hollow  conductors,  58-66. 
Hollow  cube,  Faraday's,  65. 
Holtz  machine,  the,  108-110, 122-124. 
Holtz  and  Topler  machines  compared,  122- 

124. 

Holtz,  Dr.  W.,  correspondence  with,  123, 124. 
Hydro-electro   machine,    Armstrong's,  105- 

107. 


Idiostatic  gauge  for  electrometer,  163. 
Image  plates,  103,  104. 
Induction,  43-54. 

,  theory  of,  48, 49. 

varies  inversely  as  square  of  distance,  46, 

47. 
Inductive  capacity,  specific,  51-54. 

influence  of  dielectric,  49-51. 
Influence  machines,  108. 
Insulator  defined,  6. 
Intensity,  electric,  6-8. 


Jar,  the  Leyden,  75-91. 

Jar  D,in  Topler  machine, higher  potential  of, 
139, 140. 


Leyden  jar,  the,  75-91. 

,  charged  by  cascade,  77-79. 

,  discharged  through  book,  81-84. 

,  electromotive  force  of,  77. 

,  fractured  by  overcharge,  88, 140. 

,  glass  suitable  for,  77. 

,  Lane's  unit,  101, 102. 

,  residual  charge  of,  84,  85. 

,  spontaneous  discharge  of,  88. 

,  the  atmosphere  as  a,  180, 181. 

,  with  movable  coatings,  85,  86. 
Leyden  battery,  the,  79,  80. 

,  Tyndall's  experience  with,  87. 
Lichtenberg's  figures,  89-91. 
Light,  heat,  and  electricity  compared,  26-31, 
138. 

,  polarized  and  electricity,  28-31,  3fi. 
Lightning  and  thunder,  207-223. 
Lightning  conductors,  216-221. 

,  attachment  of,  218. 

,  earth  connection  of,  221. 

,  form  Of,  219,  220. 

,  inspection  of,  C21. 


INDEX. 


227 


Lightning  conductors,  joints  of,  220. 

,  material  lor,  219. 

.points  lor,  217. 

,  silent  discharge  of,  222. 

,  space  protected  by,  217, 218. 

,  test  of  copper  tape,  221,  222. 
Lines  of  force,  55. 


M 


Machine,  Armstrong's    hydro-electric,   105- 

107. 

described  by  Xoad,  100. 
,  frictional,  96-100. 
.  the  Holtz,  108-110. 
,  the  Topler,  110-122. 
Machines  compared,  Iloltz  and  Topler,  122- 

1-24. 

,  influence,  108. 

Measurement  of  energy,  100-102. 
Medical  treatment  by  electricity,  142, 143. 
Metals  electrified  by  friction,  4,5, 132. 
Metal  screen,  inductive  action  of,  152,  153. 
Mode  of  action  of  the  f  rictional  machine,  99, 

100. 

of  the  Holtz  machine,  122,  123. 
of  the  Topler  machine,  115-124. 
Multiplication  of  charge  in  Toplei  machine, 
121, 122. 


N 

Nature  of  electricity,  23-42. 
Negative  charge,  22. 

potential,  12, 13,  21. 

sign,  13. 

Non-conductors,  4,  5,  6. 
Non  electrics,  4. 


O 


O/one,  generation  of,  131. 


I'ail  experiment,  CO-65. 
Pane,  the  charged,  72-74. 
Plates,  image,  103. 104. 
Points,  air  current  from,  104, 105. 

,  influence  of,  C9,  70. 
Polarized  light  and  electricity,  28-31, 
Proof  plnne,  58,  59. 
Positive  and  negative,  12, 13,  21. 

sign,  13. 


Potential,  atmospheric,  177-180. 
and  earth  currents,  175-189. 
,  el.-ctric,  10-22, 
,  difference  of,  11, 12. 
,  difference     of,    between    atmospheric 

strata,  179, 180. 
,  diurnal  and  seasonal  variation  of,  177- 

179. 

of  jar  P,  in  Topler  machine.  139,  140. 
.  reversal  of,  in  Tbpler  machine,  120,  140. 
,  zero,  13,  65,  66. 

Power,  transmission  of,  by  static  electricity, 
128,129. 


Q 


Quadrant  electrometer,  Thomson's,  169-174. 
Quantity  and  intensity,  6-8. 


Radiant  energy,  31. 
matter,  154. 

Replenisher  for  electrometer,  164. 

Repulsion,  1-4, 15, 16,  1C9. 

Residual  charge,  84,  85. 

Reversal  of  potential  in  Topler  machine,  120, 
140. 

Rotation  of  Topler  machine,  direct  and  re- 
versed, 138, 139. 

Rotary  movement  in  Aracua,  149-151. 


s 


Silent  discharge,  89. 

Source  of  electric  supply  of  the  Topler  ma- 
chine, 129-132. 

Spark,  the,  its  direction,  subdivision,  and 
color,  133-138. 

,  and  snap,  39,  40. 
Specific  inductive  capacity,  51-54. 
Spheres,  electrified,  68,  69. 
Spheroid,  electrified,  70,  71. 
Spontaneous  discharge,  88. 
Static  electricity  defined,  8, 9. 
Surface  condensation ,  55-58. 

,  thickness  of  electrified,  66. 

transmission ,  58. 


Telegraph  lines,  observations  on,  186-183,  204, 

205 
Tides,  electric,  183,  184. 


228 


INDEX. 


Time  of  electric  discharge,  126-123. 
Thermopile,  illustrations  from  the,  175, 176. 
Thickness  of  electrified  surface,  66. 
Thunder,  211,  214. 

clouds,  formation  of,  207-208. 
Topler  machine,  the,  110-122. 

,  the  four-plate,  114. 

,  experiments  with,  125-145. 

,  mode  of  action  of,  115-124. 
Tornadoes,  223. 
Torsion  balance,  Coulomb's,  156-161. 

,  inaccuracy  of  the,  159 101. 
Transmission,  electric,  in  vacua,  146-154. 

of  power  by  static  electricity,  118, 12'J. 

,  surface,  58. 
Tubes,  Geissler,  147, 148. 
Tube,  vacuum,  146, 147. 


u 

Universal  discharger,  87,  88. 
Unit  jar,  Lane's,  101, 102. 


Vacua,  electric  transmission  in,  32, 146-154. 

,  electric  transmission  in  low,  146-149. 

,  electric  transmission  in  high,  149-154. 

,  rotary  movement  in  high,  149-151. 
Vacuum  tube,  146, 147. 


w 

Wave  theory,  the,  31-37. 
Whirl,  the  electric,  104. 
Wind,  electric,  104, 105, 143. 


Zero  potential.  13.  C5,  66. 


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LOCKWOOD.  Electricity,  Magnetism,  and  Electric  Telegraphy. 
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pages,  152  illustrations  .......  $2.50. 

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the  Wheatstone  Automatic  Telegraph.  By  Win.  Maver,  Jr. 
8vo,  cloth,  126  pages,  63  illustrations.  .  .  .  $1.50. 

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POPE.  The  Modern  Practice  of  the  Electric  Telegraph.  Ninth 
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PRESCOTT.     Electricity  and  the  Electric  Telegraph.     8vo,  cloth, 

670  illustrations.     2  vols  .......         $5.00. 

SABINE.  History  and  Progress  of  the  Electric  Telegraph.  12mo, 

cloth,  134  illustrations  .......        $1.25. 

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WILLIAMS.  Manual  of  Telegraphy.  Cloth,  327  pages,  90  illus- 
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TELEPHONY. 

DOLBEAR.  The  Telephone.  An  account  of  the  Phenomena 
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with  Directions  for  Making  a  Speaking  Telephone.  18mo,  cloth. 
Illustrated  .........  $0.50. 

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LOCKWOOD.  Practical  Information  for  Telephonists.  New 
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PRESCOTT.  Bell's  Electric  Speaking  Telephone.  Its  Invention, 
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THOMPSON.  Philip  Reis,  Inventor  of  the  Telephone  ;  a  Bio- 
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ELECTRIC    LIGHTING. 

ALGLAVE  AND  BOULARD.  The  Electric  Light :  Its  History, 
Production,  and  Application.  Translated  by  T.  O'Connor  Sloane, 
L.M.  Edited,  with  notes  and  additions,  by  C.  M.  Lungren,  C.E. 
8vo,  458  pages,  252  illustrations $5.00. 

BOTTONE.  The  Dynamo.  How  Made  and  How  Used.  A  book 
for  amateurs.  Cloth.  Illustrated.  ....  $1.00. 

CUNYNGHAME.  A  Treatise  on  the  Law  of  Electric  Lighting, 
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DAY.     Electric  Light  Arithmetic.     32mo,  cloth.          .         $0.40. 

DREDGE.  Electric  Illumination.  Electrical  Units,  Measurement 
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neto and  Dynamo  Electric  Generators,  Conductors,  Carbons,  Arc- 
Lamps,  JablochkofE  Candle,  Incandescence  Arc-Lamps,  Incan- 
descence Lamps.  Chiefly  compiled  from  Engineering.  With- 
Abstracts  of  the  Specifications  deposited  at  the  Patent  Office 
between  1837  and  1882,  having  reference  to  Electric  Lighting, 
prepared  by  W.  Lloyd  Wise.  Numerous  illustrations.  2  vols. 
Thick  4to,  cloth.  (Sold  separately.)  Vol.  I  (scarce),  $15.00. 

Vol.  II,  $  7.50. 

DU  MONCEL.  Electric  Lighting.  Translated  by  R.  Routledge. 
Second  edition,  with  6(J  illustrations.  12mo,  cloth.  .  $1.25. 

DU  MONO  EL.  Incandescent  Electric  Lights,  with  particular  ref- 
erence to  the  Edison  Lamps  at  the  Paris  Exhibition.  To  which 
is  added  the  Economy  of  the  Electric  Light  by  Incandescence, 
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rent, by  C.  W.  Siemens.  Illustrated.  18mo,  boards.  $0.50. 

ELECTRIC  LIGHTING  ACT,  1882,  and  the  Acts  therewith  In- 
corporated, also  the  Rules  of  the  Board  of  Trade,  Oct.,  1882. 
With  introduction,  notes,  and  index,  by  W.  C.  Glen  and  A.  Glen. 
12mo,  cloth $2.00. 

GORDON.  A  Practical  Treatise  on  Electric  Lighting.  Svo,  cloth. 
228  pages.  23  plates  and  numerous  illustrations.  .  $4.50. 

HAMMOND.  The  Electric  Light  in  our  Homes.  With  original 
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IIIGGS.  Magneto  and  Dynamo  Electric  Machines,  with  a  descrip- 
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Cew,  by  F.  Krohn,  16mo,  301  pages,  60  illustrations.  $2.00. 

HOLMES.  Practical  Electric  Lighting.  12mo,  cloth.  62  illus- 
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HOPKINSON".  Dynamic  Electricity:  Its  Modern  Use  and  Meas- 
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raphy ;  including  some  I.  Points  in  Electric  Lighting  ;  II.  On 
the  Measurement  of  Electricity  for  Commercial  Purposes.  By 
J.  N.  Shoolbred;  III.  Electric  Light  Arithmetic.  By  R.  E. 
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MAIER.  Arc  and  Glow  Lamps.  A  practical  hand-book  of  Elec- 
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PRESCOTT.  Dynamo-Electricity.  Its  Generation,  Application, 
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SCIIELLEN".  Magneto-Electric  and  Dynamo-Electric  Machines; 
their  Construction  and  Practical  Application  to  Electric  Lighting 
and  the  Transmission  of  Power.  Translated  from  the  third 
German  edition  by  Nathaniel  S.  Keith  and  Percy  Neyrnann,  Ph. 
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Machine.  By  Nathaniel  S.  Keith.  Cloth,  510  pages.  353  illustra- 
tions. .........  $5.00. 

SilOOLBRED.  Electric  Lighting  and  its  Practical  Applications, 
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THOMPSON.  Dynamo-Electric  Machinery.  (New  Edition.  En- 
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Cloth,  Svo,  527  pp.,  324  illus.  New  ed.  Revised.  .  $5.00. 

THURSTON.  Stationary  Steam  Engines,  especially  as  adapted 
to  Electric  Lighting  purposes.  16mo,  177  pp.,  illus.  $2.00. 

URQUHART.  Electric  Light ;  its  Production  and  Use,  embody- 
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Electric  Lamps  and  Dynamo-Electric  Machines.  Edited  by  C. 
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WALKER.  Practical  Dynamo  Building  for  Amateurs.  Illus- 
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TRANSMISSION    OF    POWER. 

DU  MONCEL.  Electricity  as  a  Motive  Power.  Translated  and 
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KAPP.  Electric  Transmission  of  Energy  and  its  Transformation, 
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LUCE.  Electric  Railways  and  the  Electric  Transmission  of  Power. 
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Cloth,     1.00. 

MARTIN  AND  WETZLER.  The  Electric  Motor  and  its  Appli- 
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URQUHART.  Electro-Motors.  A  Treatise  on  the  Means  and 
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and  its  Conversion  into  Motive  Power.  12 mo,  cloth,  illus- 
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TESTING  AND    MEASUREMENTS. 

HASKINS.  The  Galvanometer  and  its  Uses.  A  Manual  for  Elec- 
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KOIILRAUSCH.  An  Introduction  to  Physical  Measurements, 
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LOCKWOOD.  Electrical  Measurement  and  the  Galvanometer ; 
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NIPHER.  Theory  of  Magnetic  Measurements  ;  with  an  appendix 
on  the  Method  of  Least  Squaces.  94  pages,  illus.  $1.00. 

SCHWENDLER.  Instructions  for  Testing  Telegraph  Lines. 
Second  edition.  2  vols.  8vo,  cloth,  illustrated.  .  $8.00. 

ELECTRO -METALLURGY,  ELECTROTYPING,  ELEC- 
TROPLATING. 

FONTAINE.  Electrolysis.  A  Practical  Treatise  on  Nickeling, 
Coppering,  Gilding,  Silvering,  the  Refining  of  Metals  and  Treat- 
ment of  Ores  by  means  of  Electricity.  By  Hippolyte  Fontaine. 
Translated  from  the  French  by  J.  A.  Berley.  Cloth.  264  pages. 
34  illustrations. $3.50. 

GORE.  The  Art  of  Electro-Metallurgy,  including  all  Known 
Processes  of  Electro-Deposition.  12mo,  cloth,  illustrated.  $2  25. 

NAPIER.  A  Manual  of  Electro-Metallurgy,  with  the  Applica- 
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cloth.  $3.00. 

URQUHART.  Electrotyping.  A  Practical  Manual.  12mo,  cloth, 
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URQUHART.  Electro-Plating.  A  Practical  Hand-book,  includ- 
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WAHL.  Galvanoplastic  Manipulations.  A  Practical  Guide  for 
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Operator.  Comprising  the  Electro-Deposition  of  all  Metals  by 
means  of  the  Battery  and  the  Dynamo-Electric  Machine,  etc., 
etc.  189  illustrations,  8vo,  cloth $7.50. 


WAT*!1.  Electro-Metallurgy,  Practically  Treated.  New  and  en- 
larged edition.  12mo,  cloth .$1.00. 

WATT.  Electro-Deposition.  A  Practical  Treatise  on  the  Elec- 
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Alloys,  with  descriptions  of  Voltaic  Batteries,  Magneto  and 
Dynamo  Electric  Machines,  Thermopiles,  and  of  the  Materials 
and  Processes  used  in  every  Department  of  the  Art,  and  several 
chapters  on  Electro-Metallurgy.  Cloth.  568  pages.  144  illus- 
trations  $5.00. 

WILSON".  Stereotyping  and  Electrotyping.  A  guide  for  the 
production  of  plates  by  the  papier-mache  and  plaster  processes. 
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MISCELLANEOUS. 

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BLAKESLEY.  Alternating  Currents  of  Electricity.  Cloth.  90 

pages,  11  illustrations.  .         .         .         .         .         $0.60. 

DE  FONVIELLE.  Thunder  and  Lightning.  Cloth,  285  pages, 

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DU  MONCEL.  Electro-Magnets.  The  Determination  of  the 

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EVERETT.  Units  and  Physical  Constants.  New  edition.  200 

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GLADSTONE  AND  TRIBE.  The  Chemistry  of  the  Secondary 

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GORE.  Electro-Chemistry,  Inorganic.  Cloth,  138  pages.  $0.80. 
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the  Paris  Electrical  Exhibition  of  1883.     Svo,  cloth,  287  pages, 

258  illustrations $2.00. 

HOSPITALIER.  Domestic  Electricity.  Describing  the  most 

recent  devices  in  the  application  of  electricity  to  domestic  use. 

Translated  from  the  French,  with  additions,  by  C.  J.  Wharton. 

Svo,  cloth,  229  pages,   155  illustrations.  .         .        $3.00. 


HOSPITALTER.  The  Modern  Applications  of  Electricity. 
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JEANS.  Lives  of  the  Electricians, — Profs.  Tyndall,  Wheatstone, 
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LOCK.  Workshop  Receipts.  Third  series.  Devoted  mainly  to 
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MUXRO  &  JAMIESON.  Electricians'  Pocket-Book  of  Electrical 
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SPANG.  Treatise  on  Lightning  Protection.  12mo,  cloth,  illus- 
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SWINBURNE.  Practical  Electrical  Units  Popularly  Explained. 
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WELCH.  Table  of  Relative  Weights  of  Copper  Conductors  for 
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WIG  AN.  The  Electrician's  Pocket-Book.  The  English  edition 
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WORMELL.  Electricity  in  the  Service  of  Man.  A  popular  and 
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IN  ELECTRICITY  AND  ITS  PRACTICAL 

APPLICATIONS. 


ISSUED     EVERY    SATURDAY. 


PUBLICATION  OFFICES,  168-177  POTTER  BUILDING,  NEW  YORK, 


W.  J.  JOHNSTON,    Editor  and  Publisher. 

T.  COMMERFORD  MARTIN,  \  A         .   .     .....  CLARENCE   E.  STUMP, 

JOSEPH   WETZLER,  |  Associate  Ed.tors.  Business  Manager. 


New  England  Office,     48  Congress  Street,  Boston. 
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The  Electrical  World 

Is  the  Pioneer  Weekly  Electrical  Journal  of  America,  and  has  well  maintained 
its  lead.  It  has  the  largest  circulation  of  any  periodical  in  the  world  devoted  to 
electricity,  and  is  noted  for  its  ability,  enterprise,  independence,  and  honesty. 
For  thoroughness,  candor,  and  progressive  spirit,  it  stands  in  the  foremost  rank 
of  special  journalism.  Its  low  subscription  price,  combined  with  its  acknowl- 
edged excellence,  renders  the  paper  so  popular  that  no  one  who  reads  any  elec- 
trical journal  is  willing  to  do  without  The  Electrical  World. 

It  has  no  equal  as  an  Advertising  Medium  in  its  special  field. 

Avoiding  abstruse  technicalities,  The  Electrical  World  seeks  to  keep  its  readers 
informed  of  every  event  of  importance,  every  new  discovery,  invention,  applica- 
tion, and  theory,  in  which  electricity  plays  a  part.  No  one  who  desires  to  keep 
abreast  of  the  wonderful  activity  in  electrical  discovery  and  invention  that 
characterizes  our  times,  can  afford  to  be  without  it. 

Correspondence,  news  items,  views,  and  opinions,  on  all  topics  within  the 
province  of  this  journal,  are  cordially  invited  from  any  part  of  the  world. 

Matter  for  the  Editorial  Department  should  be  addressed  to  "The  Editor  of 
The  Electrical  World,  New  York."  Subscriptions  and  communications  relating 
to  Advertising  or  the  Business  Department  should  be  addressed  to 

W.  J.  JOHNSTON,    Publisher, 
168-177  Potter  Building,  NEW  YORK. 


14  DAY  USE 

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LOAN  DEPT. 

RENEWALS  ONLY— TEL.  NO.  642-3405 
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APR  13 1981 


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General  Library 

University  of  California 

Berkeley 


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